Give the keyboard user the ability to move among all modeless windows

With realtime preview, the keyboard user will need this to get
back and forth between the effect and the project window.  Actualy,
the "problem" existed before this since the keyboard user couldn't
switch between project and Plot Spectrum windows.

The normal flags-based keyboard handling would not work since it
depends on the state of the project and requires a project window
to have the current focus.

In this new case, a modeless dialog may actually have the focus and
using the "switch window" key combo may happen in nearly any mode, like
while playing.

So, I "created" the concept of "meta" commands...ones that do not have
the restrictions mentioned above.  The should be used sparingly and
you must be careful about what happens within their handlers.

src/Menus.h

@@ -384,6 +384,9 @@ void OnSeparator();
 void PrevFrame();
 void NextFrame();
 
+void PrevWindow();
+void NextWindow();
+
 void OnResample();
 
 // Make sure we return to "public" for subsequent declarations in Project.h.