Command flag functions out of Menus.cpp, reducing its dependencies...

... Mostly into CommonCommandFlags.cpp, but some elsewhere, to avoid giving
that new file problematic dependencies on LabelTrack, ControlToolBar, and
EffectManager.

Note that CutCopyAvailableFlag is critically ordered, for message purposes,
only with AudioIONotAvailableFlag, the only flag with a message that it combines
with in menu item definitions.

The dependency on LabelTrack.cpp might not be a bad one later, if the track and
its view can be separated, and that would allow CutCopyAvailableFlag to be
put with the others.  But much other work on LabelTrack must happen first.

lib-src/mod-null/ModNullCallback.cpp

@@ -32,6 +32,7 @@ click from the menu into the actaul function to be called.
 #include "ShuttleGui.h"
 #include "Project.h"
 #include "commands/CommandManager.h"
+#include "CommonCommandFlags.h"
 
 #if defined(__WXMSW__)
 #include <wx/init.h>