gtk-layer-shell wants Gdk::Monitor instead of wl_output;
change code to deal with Gdk objects and slightly simplify it.
Requires gtkmm 3.22.0+ (first release with Gdk::Monitor support).
It seems that dbusmenu is not ready to display menu immediately and
needs some time to sync data via DBus.
Fixes LIBDBUSMENU-GLIB-CRITICAL: dbusmenu_menuitem_send_about_to_show:
assertion 'DBUSMENU_IS_MENUITEM(mi)' failed.
Also fixes initial render of the menu with layer shell popups support patch.
Set ItemIsMenu to true by default because libappindicator supports
neither ItemIsMenu nor Activate method and compiant SNI implementations
are expected to reset the flag during initial property fetch.
To be revisited if anyone finds the implementation that has Activate
but does not set ItemIsMenu.
Aleksei Bavshin