Honor locale time settings in lock screen.

The time display in the lock screen should honor the LC_TIME variable,
including the locale-specific settings for whether to display a 12-hour or
24-hour time.  Sending the time and date strings through gettext results in
a value which is specific to LC_MESSAGES, which can be different from
LC_TIME.  Use the %X value for time, which is guaranteed to be appropriate
for the locale in question.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Revert "Honor locale time settings in lock screen."

This reverts commit debbeac50a48c78fd0ca1559228e54eebc4d7aa8.
Honor locale time settings in lock screen

Closes https://github.com/mate-desktop/mate-screensaver/pull/47
Bump minimum GTK2 and GLIB versions
Remove some old code after GLIB and GTK2 minimum version bump
Dont call g_thread_init
Don't use deprecated GLib threading API.