compositor: fix mouse interactions on frames

meta_ui_window_is_widget() returns FALSE for frame windows, so we
must filter those explicitly (by letting the event go to gtk
and from there to MetaFrames). Also, for proper gtk widgets
(window menus) we want to let gtk see all events, including
keyboard, otherwise we break keynav in the window menu.
This means that having a window menu open disables keybindings
(because the event doesn't run through clutter)