pixbufcache: Use sceneQueueComplete event to simplify usage

pixbufcache: Use sceneQueueComplete event to simplify usage

Ensuring the native pixbuf cache is correct after new loaders have
been installed is tricky. This needs to be done without races
and work regardless of whether the build is from sstate or freshly
built for one or more modules.

This adds a hook into base.bbclass which is then triggered by the
code from pixbufcache. This patch is an improved version which
means base.bbclass has no pixbuf knowledge and the mechanism can
be reused in other cases.

(From OE-Core rev: b411085fefc8582d24c172db5f6610705eb44c2f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>