Ensure that we register with the session manager only once

https://bugzilla.gnome.org/show_bug.cgi?id=728934
Fix ucs2keysym prototype

https://bugzilla.gnome.org/show_bug.cgi?id=730897
introspection: add some missing Returns: (nullable) annotations
Use git.mk to maintain .gitignore.
Fix various memory leaks when checking D-Bus signatures
Unref enum type classes when we're done with them
Unref an accessible's cache when destroying it
Fix lifecycle of an accessible's cache

When a cache is referenced in response to an event, it is supposed
to be unref'd after the event handler has been called.
Release v2.13.4