Tree: added basic case-insensitive sorting by name

For this we also need another column 'id' to sync with hawk's List_dir.
Tree_active: use the id column instead

We need to use the id column instead of calculating the index using
std::distance which also saves some time since Gtk::TreeModel::Children
is a linked-list container so it wasn't a good idea to use std::distance
in the first place.