Switch GCond to using monotonic time for timed waits by introducing a
new API based on monotonic time in a gint64: g_cond_wait_until().

Deprecate the old API based on wallclock time in a GTimeVal.

Fix up the gtk-doc for GCond while we're at it: update the examples to
use static-allocated GCond and GMutex and clarify some things a bit.
Also explain the rationale behind using an absolute time instead of a
relative time.