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
%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)).