Expand the desktop entry field codes

Fixes #153 (closed).

I mark this as WIP for the moment, because further investigations in xfce code made me realize that libxfce4util/libxfce4util/xfce-miscutils.c:xfce_expand_field_codes() could be completed with the code in xfce4-panel/plugins/launcher/launcher.c:launcher_plugin_exec_parse() (expansion of %f, %F, %u, %U), so that xfce_expand_field_codes() would take one more argument.

This is an upcoming MR for libxfce4util (now merged: see libxfce4util!2 (merged)).