"External Panelize" dialog should use WListbox's change notification.

[mooffie]

Currently it uses a MSG_POST_KEY/MSG_FOCUS hack to achieve this.

Advantage:

We'll get rid of a subtle mouse bug. The use of MSG_FOCUS in the hack inadvertently makes clicking the mouse update the command. But MSG_FOCUS is actually fired before the listbox changes its item, so the command seen is the previously selected (you don't always notice this because WListbox's mouse handler uses MOU_REPEAT, which makes the problem sometime fix itself after a fraction of a second).

[mooffie]

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

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

[andrew_b]

Applied in #3569 as [755a2b820e534f810e724adc155d1561b7af8d39].