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.
6bbd88accf