Add transport menu items for scrubbing and seeking...

... This includes new always-seeking modes unlike scrubbing which can switch
to seeking and back according to the left mouse button state.

The reason for this is that visually impaired users should not be required to
click with the mouse in the track panel window to signal seeking.  But mouse
movements can still control scrubbing, because we poll the global mouse
position in the timer, not relying on events from any window object.

src/Menus.h

@@ -88,6 +88,11 @@ void OnPlayBeforeAndAfterSelectionEnd();
 void OnPlayLooped();
 void OnPlayCutPreview();
 
+void OnScrub();
+void OnScrollScrub();
+void OnSeek();
+void OnScrollSeek();
+
         // Wave track control
 
 void OnTrackPan();