Allow applications to use gtk-enable-mnemonics in 3.12

gnome-terminal is still using this setting, so we'll let
applications override it for another cycle. It is no longer
backed by a system-wide setting, though, and it will still
go away eventually.

This partically reverts 7e3a494fac62140f466fc43f6cc061215953e9b1
Allow applications to use gtk-menu-bar-accel in 3.12

gnome-terminal is still using this setting, so we'll let
applications override it for another cycle. It is no longer
backed by a system-wide setting, though, and it will still
go away eventually.

This partically reverts b26c74e5dac0b5fd869d2c8cc7a0edb6b01aeebd
Deprecate GdkEventVisibility and ::visibility-notify-event

These don't really work anymore in a composited world.
Lets make it official.
https://bugzilla.gnome.org/show_bug.cgi?id=481073
Fix up signal deprecation
Fix up builder test

Dialogs now report two children. Thats alright, adapt
the test to deal with it.
Update messagedialog a11y test
3.11.8
Bump version
Updated Traditional Chinese translation(Hong Kong and Taiwan)