Fix a critical when processing deferred messages

Due to GMainContext manipulations and reentrancy issues, it's
possible the source pointer is NULL inside the callback.
Let's avoid the critical warning by unreffing the source early
- it is kept alive by GMainContext anyway after the attach.

https://bugzilla.gnome.org/show_bug.cgi?id=724292
bus-launcher: Do not use g_setenv when initializing the bus

g_setenv is racy if threads exist, and g_dbus_connection_new_for_address
does what we want.

https://bugzilla.gnome.org/show_bug.cgi?id=724070
registryd: Remove some unused code
Add AtspitableCell, tracking the new atk interface