The unload of qmljs plugin caused qmljs code style settings
to be deleted, while core still had a pointer to them and
used it afterwards. Added unregisterCodeStyle* methods
for symmetry and used them on plugin destruction.

Change-Id: I49f0fc52f3e71d053e6ada604672a4cc3eafa486
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>