game_timing_state() now has access to the game_ui. This means that

game_timing_state() now has access to the game_ui. This means that
whether the timer is currently going is no longer solely dependent
on the current game_state: it can be dependent on more persistent
information stored in the game_ui. In particular, Mines now freezes
the timer permanently once you complete a grid for the first time,
so that you can then backtrack through your solution process without
destroying the information about how long it took you the first time
through.

[originally from svn r6088]