Add a Toolbars settings page: icon size, text with icons, lock

A new page in Configurer QElectroTech, "Barres d'outils", sets for
every window (folio editor, element editor, title block editor):
- the icon size: default, 16, 24, 32 or 48 px
- the buttons: icon only, text beside the icon, or text under it
- locking the toolbars so they can't be dragged out of place

OK applies it to the open windows at once. The defaults are Qt's
own, and a default value removes its key, so nothing changes for
anyone who never opens the page. A toolbar inside a panel is left
alone; only the window's own toolbars are locked.

New test tst_toolbarsettings: defaults change nothing, saved values
reach every toolbar of an open window, back to defaults removes the
keys.

Third stage of discussion #1319.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/qetdiagrameditor.cpp

@@ -85,6 +85,7 @@
 #include "wiringlistexport.h"
 #include "ui/wiringlistdialog.h"
 #include "ui/terminalnumberingdialog.h"
+#include "toolbarsettings.h"
 #include <QDateTime>
 #include <QDebug>
 #include <QDir>
@@ -186,6 +187,7 @@ QETDiagramEditor::QETDiagramEditor(const QStringList &files, QWidget *parent) :
 	readSettings();  // restoreGeometry before show()
 	show();
 	readSettingsState();  // restoreState() must be called after show() in Qt6
+	ToolbarSettings::applyTo(this);
 #ifdef QET_HAS_SCRIPTING
 	setUpLiveIndicator();
 	setUpMacroRecorder();