Scripting: qet.currentFolio(), and one undo step per script run in the editor

A script started from the editor had no way to know which folio is on
screen, and each qet.* call was its own undo step, so a script that adds
twenty items needed twenty Ctrl+Z to take back.

qet.currentFolio() returns the folio shown in the editor; through --run,
which has no view, the first folio, or -1 when there is none.

A run from the editor is now one undo macro, named after the script. A
run that changed nothing leaves no empty entry behind. qet.undo() and
qet.redo() inside a grouped run say why they cannot work (QUndoStack
ignores them inside a macro) instead of failing silently. --run is
unchanged: one step per call, so scripts that call qet.undo() keep
working.

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

sources/scripting/qetscripting.h

@@ -51,8 +51,9 @@ namespace QetScripting {
 
 	/**
 		@brief Run @p scriptPath against an already-open @p project (the
-		"Run Script..." GUI macro path). Errors go to stderr; there is no
-		modal reporting in this first version.
+		"Run Script..." GUI macro path). Errors go to stderr, and with a
+		@p view also to a message box. With a @p view the whole run is one
+		undo step; without one, each call is its own step, as on --run.
 		@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).