Global keyboard capture handling into new files, better comments...

... and break its compile dependency on CommandManager.h by letting it install
callbacks.

This also removes Objective-C mixed code from CommmandManager.

This also eliminates four inclusions of Project.h!

Capture handler state is also global, not per project, though the
CommandManager's callbacks still do depend on the active project.

src/TrackPanel.cpp

@@ -68,6 +68,7 @@ is time to refresh some aspect of the screen.
 #include "Experimental.h"
 
 #include "AdornedRulerPanel.h"
+#include "KeyboardCapture.h"
 #include "Project.h"
 #include "TrackPanelMouseEvent.h"
 #include "TrackPanelResizeHandle.h"
@@ -2294,7 +2295,7 @@ void TrackPanel::SetFocusedTrack( Track *t )
    auto cell = mAx->SetFocus( Track::SharedPointer( t ) ).get();
 
    if (cell) {
-      AudacityProject::CaptureKeyboard(this);
+      KeyboardCapture::Capture(this);
       Refresh( false );
    }
 }