dzl_g_date_time_format_for_display: Use the new "%OB" format
The correct way to display a month name standalone is "%OB" now. This is since the GLib bug https://bugzilla.gnome.org/show_bug.cgi?id=749206 has been fixed which follows the glibc bug https://sourceware.org/bugzilla/show_bug.cgi?id=10871.
That means that from now libdazzle requires GLib >= 2.55.3.
Closes: #8 (closed)