"Directory hotlist" dialog should use WListbox's change notification.

[mooffie]

The directory hotlist updates the "Directory path" label when the item in the listbox changes.

It uses MSG_POST_KEY to detect this change.

We should instead use WListbox's notification mechanism: it sends a MSG_ACTION in such case.

The patch fixes this.

Advantage:

The dialog will be able to detect changes caused by mouse clicks too. Currently it can't.

[mooffie]

Note that after tending to ticket:3566 the code will be tidier:

case MSG_INIT:
case MSG_NOTIFY:
    uupdate_path_name (); 
    return MSG_HANDLED;

[andrew_b]

Applied in #3569 as [e7fdb5500514e0d4a1da7e2fcf5e72ae6e841780].