Removing focus from child windows upon closing them.

When any of Song Editor, BB Editor, Automation Editor, or Piano Roll, are closed (technically toggled off or "hidden"), they retain focus and capture key events.  This can be demonstrated by loading a song into the Song Editor and closing all other child windows, focusing the Song Editor window, hitting [space] to start playback, then closing/hiding the Song Editor (using the window close button, the Song Editor toggle menu button, or the F5 key), and then pressing [space] again while no child windows are open.  It is then observed that the key events continue to be handled by the child window, toggling playback.  This PR restores focus to the parent window in this situation.