Add configurable shortcuts: ShortcutManager registry + Shortcuts config page (#574)

Implements the first pillar of #574: a "Shortcuts" preferences page letting
users rebind, search and reset every keyboard shortcut in the app.

What it does
- New ShortcutManager singleton: every one of the ~95 setShortcut()/
  setShortcuts() call sites across qet.cpp, qetmainwindow.cpp,
  elementspanelwidget.cpp, autonumberingdockwidget.cpp, richtexteditor.cpp,
  qetdiagrameditor.cpp, qettemplateeditor.cpp and qetelementeditor.cpp now
  calls registerAction(target, id, category, default_sequence) instead,
  which applies the user's saved override (or the default) and remembers
  the target for later editing.
- New ShortcutsConfigPage, added to the existing "Configurer QElectroTech"
  dialog: a filterable table of every registered shortcut, grouped by
  category, each with a QKeySequenceEdit and a per-row reset button, plus a
  "reset all" button. Bindings are only persisted (via
  ShortcutManager::setSequence()) when the dialog is accepted.
- Conflict detection: rows whose currently-edited sequence collides with
  another row are highlighted with a tooltip naming the conflicting action.
- Overrides are stored under a "shortcuts/" QSettings group, one key per
  id, keyed to match the id (not persisted at all when equal to the
  hardcoded default), so a future QET version can safely raise a default
  for anyone who never customized it.

Design notes
- Targets are handled generically via QObject rather than QAction, since one
  call site (autonumberingdockwidget's "Configurer" button) is a
  QPushButton, not a QAction. Both declare an identical "shortcut"
  QKeySequence Q_PROPERTY, so registerAction() reads/writes it through the
  property system instead of needing a separate code path.
- Several live targets can share one id at once -- QET allows multiple
  windows of the same kind (diagram editor, element editor...) open
  simultaneously, each constructing its own QAction with the same id.
  setSequence() updates every live target for that id in one call, so a
  rebind takes effect in all open windows immediately, without restart.
- A shortcut's description is captured from its target's text() the first
  time that id is registered, then cached -- so the config page stays
  correct even after the owning window is closed. One consequence: a
  shortcut belonging to an on-demand window (element editor, title block
  editor, rich text editor) only appears in the list once that window has
  been opened at least once in the current session, since nothing has
  registered its id yet otherwise.

Testing
Full CMake build (qmake CONFIG+=no_kf5, Qt 5.15) compiles clean with zero
errors and zero new warnings. Verified end-to-end in a real running session
(Xvfb + xdotool):
- The Shortcuts page appears in Configure QElectroTech with the right icon,
  lists every always-registered shortcut with correct category/action name/
  current binding.
- The filter box correctly narrows the list, and correctly returns nothing
  for an action whose owning window hasn't been constructed yet this
  session (confirming the on-demand-registration behavior above is working
  as designed, not silently broken).
- Conflict detection correctly flagged a real pre-existing same-key overlap
  between "Supprimer" (delete selection, Del) and "Supprimer ce folio"
  (delete diagram from panel, Del) -- both highlighted with explanatory
  tooltips.
- Rebound "Manuel en ligne" to Ctrl+Shift+M, clicked OK: persisted under
  [shortcuts] in QElectroTech.conf, and the Aide menu's entry showed the new
  binding immediately, no restart needed.
- Reopened the dialog: the rebind was still shown. Clicked its per-row
  reset button, then OK: the settings key was removed entirely (not stored
  as "F1"), correctly falling back to the hardcoded default.

Retrofitting the Tab/Shift+Tab, select-all (#585) and Ctrl+G jump-to-element
(#586) shortcuts through this registry is left for a follow-up once those
PRs land, to avoid re-merging still-open branches into this one.

Developed with assistance from Claude (Anthropic).

sources/richtext/richtexteditor.cpp

@@ -47,6 +47,7 @@
 
 #include "richtexteditor_p.h"
 #include "ui_addlinkdialog.h"
+#include "../shortcutmanager.h"
 
 //#include <QtDesigner/QDesignerFormEditorInterface>
 
@@ -461,19 +462,19 @@ RichTextEditorToolBar::RichTextEditorToolBar(RichTextEditor *editor,
 	m_bold_action = createCheckableAction(
 				QIcon(":/ico/32x32/format-text-bold.png"),
 				tr("Texte en gras"), editor, SLOT(setFontBold(bool)), this);
-	m_bold_action->setShortcut(Qt::CTRL | Qt::Key_B);
+	ShortcutManager::instance().registerAction(m_bold_action, "richtext.bold", tr("Éditeur de texte"), Qt::CTRL | Qt::Key_B);
 	addAction(m_bold_action);
 
 	m_italic_action = createCheckableAction(
 				QIcon(":/ico/32x32/format-text-italic.png"),
 				tr("Texte en italique"), editor, SLOT(setFontItalic(bool)), this);
-	m_italic_action->setShortcut(Qt::CTRL | Qt::Key_I);
+	ShortcutManager::instance().registerAction(m_italic_action, "richtext.italic", tr("Éditeur de texte"), Qt::CTRL | Qt::Key_I);
 	addAction(m_italic_action);
 
 	m_underline_action = createCheckableAction(
 				QIcon(":/ico/32x32/format-text-underline.png"),
 				tr("Texte souligé"), editor, SLOT(setFontUnderline(bool)), this);
-	m_underline_action->setShortcut(Qt::CTRL | Qt::Key_U);
+	ShortcutManager::instance().registerAction(m_underline_action, "richtext.underline", tr("Éditeur de texte"), Qt::CTRL | Qt::Key_U);
 	addAction(m_underline_action);