icon-theme: Clean up locking

icon-theme: Clean up locking

Move the lru cache under the global cache lock to avoid some ABBA
style deadlocks when going from icon_theme->icon lock an icon->icon_theme.
We also move all the icon lock uses to a small part of code and make
sure that code never calls out or blocks with any locks held.

Rename the GtkIcon->cache_lock to texture_lock to avoid confusion withe
the global cache_lock.

Removed any mentions of threadsafety from the API docs, we don't
want apps to rely on this, but rather use it outselves internally.