Smarter SelectAllIfNone

Now if there is a time selection and no tracks selected, then just select all the tracks, preserving the time selection.
This helps in the case a user has made a time selection, e.g. with selection toolbar, and then clicked on track panel, losing the selection of tracks but preserving the time selection.

I also shortened some repeated cut-and-pasted code.

src/Menus.h

@@ -276,6 +276,7 @@ void OnJoinLabels();
 void OnDisjoinLabels();
 
 void OnSelectAll();
+void OnSelectSomething();
 void OnSelectNone();
 #ifdef EXPERIMENTAL_SPECTRAL_EDITING
 void OnToggleSpectralSelection();