Fix for EXT-8025 ('+' button on My Appearance > Edit Outfit panel does nothing)

Button now gets disabled in the early-exit case in LLPanelOutfitEdit::onInventorySelectionChange().

onInventorySelectionChange() also uses the new predicate get_can_item_be_worn(), which both checks whether the item is already being worn and whether it's of a type that could be worn.

Reviewed by Nyx at http://codereview.lindenlab.com/2451030 .