main: Make sure that --mode parameter is valid

Instead of falling back to a set of default values or crashing the
window manager when an invalid mode is specified, check the value
of the ShellGlobal:session-mode property before taking over as WM
and make a clean exit if it cannot be resolved to an existent mode.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

src/shell-global-private.h

@@ -13,6 +13,7 @@ void _shell_global_set_plugin      (ShellGlobal  *global,
 
 GjsContext *_shell_global_get_gjs_context (ShellGlobal  *global);
 
+gboolean _shell_global_has_valid_session_mode (ShellGlobal *global);
 gboolean _shell_global_check_xdnd_event (ShellGlobal  *global,
                                          XEvent       *xev);