use KCountryFlagEmojiIconEngine for regional flag icons

the old pixmap approach was deprecated throughout all of kf5 already.
it's replaced by the iconengine in kf6

this features a very simplistic caching system. after some testing with
an async threaded cache system, a pre-caching albeit blocking system,
and this simplistic system it appears that the simplest approach has the
best code-to-performance ratio. more involved caching brings only
marginal improvements