visualgst: Improve the debugger by highlighting code

Center the view and highlight the current line of code.

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

	* DebugTools.st: Add >>#receiver for VisualGST.

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

	* Debugger/GtkDebugger.st: Center the view and select one line.
	* Text/GtkTextWidget.st: Center the view.