Move QssParser out of UiStyle

After the design of the new styleengine is becoming clearer to me, I've decided
to move QssParser out of UiStyle into the global namespace (and its own pair of files).
QssParser has now adapted to the new message label concept, and can determine both
format code and message label from a ChatLine section header.

This is still mostly untested and quite unfinished work in progress.