Memory and file descriptor leak in QFontCache

Make the cache also use the ref counts
Make everyone who decrements a ref count check for 0 and delete
Move all cache logic to the cache
Same idea as 36cb3f3 and b3dae68 in Qt 5 without the extra stuff

Task-number: QTBUG-38035
Change-Id: I27bea376f4ec0888463b4ec3ed1a6bef00d041f8
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>