Previously, we stored the starting time in the PGN in the standardized
TimeControl field. However this is not sufficient for restarting games
in progress, as the time remaining for each player is not stored. So we
must extend PGN to store the time remaining for each player, and look at
that when starting a game. (Custom PGN extensions are kosher; that's
what we're already doing to store opposing AI and difficulty.)
https://bugzilla.gnome.org/show_bug.cgi?id=701579
This commit means that gnome-games will now build with Vala 0.15, which
is the version of Vala that all the other GNOME modules are targetting
(and will have a stable release alongside GNOME 3.4).
It also means that older versions of Vala will no longer work.
