Support closing gst-launch windows with shortcut

Please support closing a gst-launch window with a shortcut such as Ctrl+Q. Pressing that should close the window and kill the underlying process.

Example is gst-launch-1.0 videotestsrc ! xvimagesink

Implementing this will enable users to more quickly work with gst-launch, especially in window managers such as GNOME where Alt+F4 has been phased out.