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

docs/reference/glib/glib-sections.txt

@@ -1551,6 +1551,9 @@ g_date_time_to_utc
 
 <SUBSECTION>
 g_date_time_format
+
+<SUBSECTION>
+g_date_time_source_new
 </SECTION>
 
 <SECTION>