gdatetime: Add g_date_time_source_new()

gdatetime: Add g_date_time_source_new()

Several different codebases in GNOME want to implement wall clocks.
While we could pretty easily share a private library, it's not a
substantial amount of code, and GLib already has a lot of the
necessary system-specific detection and handling infrastructure.

Note this initial implementation just wakes up once a second in the
cancel_on_set case; we'll add the Linux-specific handling in a
subsequent commit.

https://bugzilla.gnome.org/show_bug.cgi?id=655129