Update the documentation and users of this function to handle
the future case that that we have some internal decorations to the window and
useable allocation is thus smaller.
By having a separate out parameter there is no need to have an in/out function
and allows for greater robustness.
The current implementation simply returns the allocation provided.
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.
