Improve (modestly) the performance of font metrics caches

Improve (modestly) the performance of font metrics caches

This fixes two performance issues and improves the performance of
TextMetrics::redoParagraph by 15% in a workload that uses the cache a
lot. The difference will be much less when the cache is not used much.

1/ repetion of the hash code computation

The code
  if (cache.contains(key))
  	result = cache[key]:
is not efficient, since qHash(key) has to be computed twice.
To fix this a new Cache::object_str() method is added, which allows
  if (auto * obj = cache.object(key))
  	result = *obj;

2/ code of has code computation

Instead of using a verbose string that is complicated to build as
key, new key structs BreakAtKey and TextLayoutKey are introduced,
along with the relevant qHash() implementation.