Avoid unnecessary updates when using in-process cache

Whenever an in-process cache is updated, it will emit
itemsAvailable() signals to all listening glyph caches. This will
in turn cause each of the glyph caches to update and each
of the glyph nodes to be preprocessed (the entire scene graph will
be updated.) This happens even if the changes to the in-process
cache are requested by an external client, due to a cross-process
cache sharing mechanism. However, itemsAvailable() signals are
only interesting if the items were requested by the in-process
cache.

We therefore add a mechanism now to check if the glyphs were actually
requested by the cache before updating anything.

Change-Id: I529f94b3928c2a5e06fec354014fa11d21671057
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>