KDevelop, Kate and probably other applications too, recreate
KColorScheme instances repeatedly. This was very costly since we
ended up reparsing the internal color scheme configuration file
every time - the shared configuration wasn't stored anywhere thus
it's refcount dropped to zero after once the KColorScheme was
fully constructed.
Optimize this apparently common scenario by caching the configuration
in a thread_local variable and only open a new configuration when the
user changed the application color scheme.