UI: Fix transform shortcuts with multiple items selected

Firstly, removes the UpdateTransformShortcuts() method introduced in
c33fa8b which was trying to re-implement the behavior of
UpdateEditMenu().
Secondly, updates UpdateEditMenu() to account for sources without video.
Sources without video shouldn't be able to have their transform edited,
copied, pasted, or changed in any other way (because they don't have
one).

UI/window-basic-main.hpp

@@ -672,8 +672,6 @@ private:
 
 	bool restartingVCam = false;
 
-	void UpdateTransformShortcuts();
-
 public slots:
 	void DeferSaveBegin();
 	void DeferSaveEnd();