support application menu forwarding in-window as a fallback mode

[Antoine Martin]

If an OS or DE does not support global menus (ie: win32, LXDE, etc..) then we could handle this ourselves by adding a menu bar to the window and just offset the window contents.

This means using a widget inside the window, which could also be used for honouring the window gravity attribute, and may also fix windows being grabbed with KDE (r773).

For the html5 client, this will require #1471

This may impact: #919, #1034