FM: cosmetic fix, improve redability
FM: cleanup in dirview_mkdir, free all strings
FM: use g_build_path in get_full_dirname_iter

This is cleaner and ensures that we don't have trailing "/"
which can lead to glitches in other functions such as
g_path_get_dirname().

Also make sure everything is freed.
FM: add missing gtk_tree_path_free
FM: implement "rename dir" in right-click menu