Add a "toggle_recording" keybinding

Add a keybinding (defaulting to <Control><Shift><Alt>r) to record
a screencast of the session. This isn't hooked up to anything in
metacity itself, but a plugin can connect to a signal on MetaScreen.

keybindings.c all-keybindings.h: Add the keybinding
screen.c: Add a ::toggle-recording signal

http://bugzilla.gnome.org/show_bug.cgi?id=575290

src/core/keybindings.c

@@ -2853,6 +2853,17 @@ handle_panel (MetaDisplay    *display,
   meta_error_trap_pop (display, FALSE);
 }
 
+static void
+handle_toggle_recording (MetaDisplay    *display,
+                         MetaScreen     *screen,
+                         MetaWindow     *window,
+                         XEvent         *event,
+                         MetaKeyBinding *binding,
+                         gpointer        dummy)
+{
+  g_signal_emit_by_name (screen, "toggle-recording");
+}
+
 static void
 handle_activate_window_menu (MetaDisplay    *display,
                       MetaScreen     *screen,