BamfLegacyScreen: When in Unity monitor for compiz ClientMessage's to show action menu
If we get a _COMPIZ_TOOLKIT_ACTION client message with the internal value _COMPIZ_TOOLKIT_ACTION_WINDOW_MENU, then we are requested by compiz to show the window action menu, and thus let's ask the proper BamfLegacyWindow to do that, thanks to libwnck. Fixes: https://bugs.launchpad.net/bugs/1280042.