Script buttons: stored scripts become commands with an icon

Every .js file in the "scripts" folder of the user's data folder that
starts with a // ==QETScript== header becomes a command: in Projet >
Scripts, as a button on a new Scripts toolbar, and, because it is
registered with ShortcutManager as diagrameditor.script.<file name>, in
the shortcut settings, the shortcut bar (S) and command search. The
header gives its name, icon (a file next to the script or a theme icon;
a tile with its initials otherwise), tooltip, default shortcut and when
it is enabled (always, with a selection, with a conductor selected).

The folder is watched, so a script added, edited or deleted while QET is
open appears, changes or goes without a restart. A file with a header
that cannot be used gets no button; the Scripts menu lists it with the
reason. The menu also opens the folder, and holds "Exécuter un script…".

A click runs the script on the current project as one undo step named
after it, and asks to switch scripting on first, like "Exécuter un
script…" does: scripting stays off by default.

ShortcutManager::unregisterAction() takes a command out of the lists
when its script is deleted, and lets it come back under a new name.

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

sources/scripting/qetscripting.h

@@ -57,9 +57,12 @@ namespace QetScripting {
 		@param view the active DiagramView, so the script's zoom methods
 		have something to act on; nullptr from the headless entry point,
 		where they become no-ops (see QetScriptApi).
+		@param title what the undo step is called; the script's file name
+		if empty.
 		@return true if the script ran without throwing.
 	*/
-	bool runOnProject(const QString &scriptPath, QETProject *project, DiagramView *view = nullptr);
+	bool runOnProject(const QString &scriptPath, QETProject *project, DiagramView *view = nullptr,
+			  const QString &title = QString());
 
 }