Fix wrong return value in x11 event dispatch

According to glib docs the dispatch function should be G_SOURCE_REMOVE
or G_SOURCE_CONTINUE. Since this was worked properly until now I assume
we were lucky enough that true == G_SOURCE_CONTINUE but it may not
always be the case.

src/x11/x.c

@@ -253,7 +253,7 @@ gboolean x_mainloop_fd_dispatch(GSource *source, GSourceFunc callback, gpointer
                         break;
                 }
         }
-        return true;
+        return G_SOURCE_CONTINUE;
 }
 
 /*