GLib 2.28 will deprecate GTimeVal and related API in favour of
standardizing on microseconds granularity for all time-based API.

Clutter should switch too.

All of the current users of GTimeVal convert to milliseconds when
doing time operations, and use GTimeVal only as storage. This can
effectively be replaced by a gint64.

The Master Clock uses a microsecond resolution, except when interacting
with the main loop itself, since the main loop has a millisecond
resolution - at least until Ryan Lortie manages to switch that too to
microseconds on Linux.

The clutter_timeline_do_tick() function was erroneously not privatized,
but it was still assumed to be private; we should just remove it from
the public symbols.