util: Add createTimeLabel()

util: Add createTimeLabel()

On locales that support it, time formats should follow the 12-hour/24-hour
preference, which implies that they should be updated when the setting
changes. So add another utility method which creates a label for a specific
time and keeps it in sync with the format setting.

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