Introduce a new game backend function (there seem to have been a lot

Introduce a new game backend function (there seem to have been a lot
of these recently) whose job is to update a game_ui to be consistent
with a new game_state. This is called by midend.c in every situation
where the current game_state changes _other_ than as a result of
make_move (Undo, Redo, Restart, Solve).

The introduction of this function allows a game_ui to contain
information about selections or highlights within a game_state which
simply wouldn't make sense when transferred to another game_state.
In particular, I've used it to fix a subtle bug in Solo whereby,
although you couldn't right-click to pencil-mode highlight a filled
square, you could _get_ a pencil-mode highlight in a filled square
if you used Undo and Redo. (Undo to before the square was filled,
right-click to highlight it, then Redo. Alternatively, left-click
and clear the square, right-click to highlight it, then Undo.)

[originally from svn r5912]