emblems api

[Nick Schermer]

For ThunarxFileInfoIface we only need to add a virtual method void
(*add_emblem) (ThunarxFileInfo *file_info, const gchar *emblem_name);
including a function thunarx_file_info_add_emblem(), maybe a
remove_emblem call too, that probably makes sense for the vcs plugin.

I have no idea if thunar looks in the xdg emblems folder, but that's a
separate issue, another DBus service is really too much for something
like this IMHO.

Nick