GtkHeaderBar: Add the titlebutton style to the close button

This will let the theme unify the appearance of this button with
the window buttons in 'traditional' title bars.
Add new api to the docs
Fix distcheck

We need to clean up the ui.h files somewhere if they are generated
during the build from tarball. Reuse the existing distclean-local
hack for it.
3.9.12
Post release version bump
Fix minor typos
Updated Polish translation
Updated Polish translation
clipboard: fix build without wayland

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>
filechooser: Don't use an underline mnemonic in the 'Save in folder' label

This used to point to the GtkPathBar, which doesn't accept mnemonic activation, anyway.
This whole thing was a leftover from when we had a combo box to select a folder, but
this is no longer the case.

https://bugzilla.gnome.org/show_bug.cgi?id=706448

Signed-off-by: Federico Mena Quintero <federico@gnome.org>