util: Add formatTime() helper method

util: Add formatTime() helper method

Displaying a time is more complex than it appears at first glance:
it should respect the user's choice regarding 12- our 24-hour format (but
only when supported by the locale) and follow the LC_TIME rather than the
LC_MESSAGES setting.
So rather than getting it more or less right in various places, it makes
sense to defer to a helper method which hopefully does the right thing. The
method added by this patch is based on _formatTimestamp in telepathyClient
with some minor tweaks:
 - there's an additional params parameter which allows enforcing
   a time-only format, even on dates other than the current one
 - only a single desktop settings object is created and shared between
   invocations

https://bugzilla.gnome.org/show_bug.cgi?id=745111