visualgst: Do not display exception handling frames in the debugger

Make the debugger skip frames that belong to the system exception
handling. This way a simple '3 halt' will stop in the frame that
signals the exception, e.g. Halt(Exception)>>signal.

2013-06-18  Gwenael Casaccio  <mrgwen@gmail.com>

	* Debugger/Extensions.st: Added.
	* Debugger/GtkDebugger.st: Skip exception handling frames.
	* Extensions.st: Remove the debugger extension.
	* package.xml: Update