API CHANGE: Allow passing of an explicit map of dropdown items to a TreeDropdownField.
BUGFIX: Make use of the $this->value setting of TreeMultiselectFields. (from r95910)
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/branches/2.4@98101 467b73ca-7a2a-4603-9d3b-597d59a354a9