We really don't want to post these events unless the *selected* items
were actually modified, since it results in a redraw of the properties
panel (which disturbs edit state and causes flicker on MSW).
Now the COMMIT system is responsible for sending this event iff the
commit touches items that are selected.
Side note: UpdateMsgPanel is useless on every app except pl_editor, so
it can probably be refactored/removed at some point.
