Do not mark stones as updated in the matrix

The GUI code checks whether stones need to be updated on screen anyway.
Make the class Matrix responsible for marking territory
Avoid double redraws of the board

On my system this brings down the CPU usage when moving the mouse
across the board considerably (from ~50% to ~2%).
Store stones and marks in the first 7 bits of the matrix entries
Do not cache groups

The time needed to assemble groups etc. is dwarfed by the time spent drawing,
so what's the point of caching them and having to worry about cache validity?
Fix capture count

I overlooked that the capture count was not local for the
Tree::addMove() function.
Remove lecacy cruft in Tree class