Showing mnemonics immediately on modifier press can be annoying and
distracting when the user is just trying to Alt+Tab into another
application/window since the mnemonic will show up and quickly vanish
again when we receive the focus out event.
https://bugzilla.gnome.org/show_bug.cgi?id=672431
In the process of removing all sealed members from headers.
At the same time, add a gtkwindowprivate.h header and move
all internal functions from gtkwindow.h there.
