Revert "Use the correct ratio character for the time label"

This reverts commit da695c6cc43db4c52a4410aa2d8e743811cef949.
Use colons to compute time label width

For 3.12 only, let's stick to colons for the time label. This means we
should also use colons for computing the width of the drawing area. Is
this likely to make any difference in the width? Probably not, but
what's the point in being wrong?
Partially revert "remove unnecessary size groups"

The header bar size group was actually important: it was keeping the New
Game button at the proper height. And since it wasn't causing any
problems, let's just put it back.

https://bugzilla.gnome.org/show_bug.cgi?id=726951