Use GtkApplication inhibit API

This will help prevent users from inadvertently loosing data when logging out
with unsaved files.

Based on: 05918e07ce

xed/xed-window-private.h

@@ -107,6 +107,8 @@ struct _XedWindowPrivate
     gint            height;
     GdkWindowState  window_state;
 
+    guint inhibition_cookie;
+
     gint side_panel_size;
     gint bottom_panel_size;