Window: Focus custom titlebar with F10

Window: Focus custom titlebar with F10

As discussed on desktop-devel-list [1], "There should be an intuitive,
consistent, immediate way to jump to the widgets that live in the
header bar." F10 has been suggested for this as it is already used to
active menubars.

F10 will focus the custom titlebar widget if the window has one and it
isn't already focused. If the titlebar widget doesn't exist or is
already focused then F10 focuses the menubar if there is one.

[1] https://mail.gnome.org/archives/desktop-devel-list/2014-February/msg00176.html

https://bugzilla.gnome.org/show_bug.cgi?id=725141