Fix for TrackPanel not releasing keyboard capture when losing focus.

Previously the TrackPanel didn't release keyboard capture when losing focus.
This led to bugs of this nature, which are now fixed:
1. On a label track, move the cursor to a position where there isn't a label.
2. Press ctrl+shift+f6 to move to the toolbars.
3. Press R. A label is created in the label track, rather than recording started.

src/TrackPanel.cpp

@@ -10145,6 +10145,10 @@ void TrackPanel::OnSetFocus(wxFocusEvent & WXUNUSED(event))
 
 void TrackPanel::OnKillFocus(wxFocusEvent & WXUNUSED(event))
 {
+   if (AudacityProject::HasKeyboardCapture(this))
+   {
+      AudacityProject::ReleaseKeyboard(this);
+   }
    Refresh( false);
 }