Big internal rewrite of code moving tiles between boards.
Now we only allocate a new Tile() as seldom as possible, instead of destroying them in the old place and rebuilding them in the new place, we reparent them.

Several variables which were holding tileNames now hold a real Tile.

This mostly seems to work but still needs some fixes, especially for the game scoring mode.

All this is in preparation for animating tile movements using QPropertyAnimation