FIX Unset internal metadata on TreeDropdownField setValue()

Otherwise the hidden field value and metadata will get out of sync.
Most commonly used together with setTitle('') as well.
FIX Don't silently unset ParentID in "add page" dialog

Happened when changing the page type, because the "selectedNode"
property isn't set in all cases
API Make Object::config use late static binding

Can now be used in instance scope, like:
  $this->config()->db
and in static scope, like:
  Page::config()->db