Recent document support

[Jani Monoses]

> GTK+ 2.10's GtkRecent* and GLib 2.12's GBookmarkFile are XBEL, but the
> current standard is not. Unfortunately. Well, GNOME people...

I think what matters is what they use in Gnome/KDE etc. as the fd.o spec
pages are sometimes orphaned. For 2.16 it looks like Gnome apps start
using the GtkRecent support in 2.10.(gnome-panel HEAD is one example)

Instead of supporting both this and the legacy ~/.recently-used
format Xfce could just go with the new one. Otherwise Xfce apps will be
'legacy' themselves and a reason for the new way not catching up quickly
enough IMHO.

Whether the best way to do this is to copy GtkRecent to frap or #ifdef
on gtk >=2.10 in the code I don't know.

Jani