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/editor/ui/qetelementeditor.cpp

@@ -54,6 +54,7 @@
 #include "../../dxf/dxftoelmt.h"
 #include "../../qet_elementscaler/qet_elementscaler.h"
 #include "../UndoCommand/openelmtcommand.h"
+#include "../../toolbarsettings.h"
 
 #include <QSettings>
 #include <QActionGroup>
@@ -89,6 +90,7 @@ QETElementEditor::QETElementEditor(QWidget *parent) :
 	readSettings();  // restoreGeometry before show()
 	show();
 	readSettingsState();  // restoreState() must be called after show() in Qt6
+	ToolbarSettings::applyTo(this);
 }
 
 /**