Fix memory leak with the shared distance field glyph cache.

Fix memory leak with the shared distance field glyph cache.

A new glyph node instance registers its owner element with its
glyph cache which in the case of the shared distance field glyph cache
connects a signal from the cache to a slot on the owner, changing the
text creates a new node but destroying the old node didn't remove the
connection causing them to accumulate over time.

In the glyph node; unregister the owner element from the cache on
destruction, and in the cache only connect an owner element the
first time it is registered and keep a reference count so the
items can be disconnected when registrations from all nodes have
been cancelled.

Change-Id: Ibf32a146e146dbbf4e0556d1bd756465bd8e45ba
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>