First cut at a game timer. Yet another backend function which

First cut at a game timer. Yet another backend function which
indicates whether a particular game state should have the timer
going (for Mines the initial indeterminate state does not have this
property, and neither does a dead or won state); a midend function
that optionally (on request from the game) prepends a timer to the
front of the status bar text; some complicated midend timing code.

It's not great. It's ugly; it's probably slightly inaccurate; it's
got no provision for anyone but the game author decreeing whether a
game is timed or not. But Mines can't be taken seriously without a
timer, so it's a start.

[originally from svn r5866]