* src/filetree.{cc,h} (find_files): Use std::string instead of Glib::ustring
for 'dirname' and get rid of the call to Glib::filename_from_utf8().  This
doesn't really belong here.

* src/mainwindow.cc (on_find_files): Convert the directory name to filename
encoding, and use the current directory as fallback if it's empty.