Revert "GObject: prevent installing properties after init"

This reverts commit ddb0ce14215cd62c7a2497d6cf9f2ea63c40ebb5.

Conflicts:
	gobject/gobject.c

https://bugzilla.gnome.org/show_bug.cgi?id=698614
docs: warn about installing properties after init

Leave ourselves a little wiggle room: if people install properties after
initialisation then we reserve the right to handle that in a way that
may not be threadsafe.

https://bugzilla.gnome.org/show_bug.cgi?id=698614