display: Unset grab_frame_action in meta_display_end_grab_op

Otherwise, if the user had moved the windows around using a mouse,
they can not be moved beyond the top edge of the primary monitor when
the tray needs to be brought up, . This is because
constrain_titlebar_visible looks at grab_frame_action and thinks that
the movement is being caused by the user trying to move the windows up
by grabbing the frame.

Fixes: https://bugzilla.gnome.org/677215

src/core/display.c

@@ -3764,6 +3764,8 @@ meta_display_end_grab_op (MetaDisplay *display,
   g_signal_emit (display, display_signals[GRAB_OP_END], 0,
                  display->grab_screen, display->grab_window, display->grab_op);
 
+  display->grab_frame_action = FALSE;
+
   if (display->grab_window != NULL)
     display->grab_window->shaken_loose = FALSE;