Unequipping Conjured Items

My first contribution to the OpenMW project. This is intended to resolve the following issue: https://bugs.openmw.org/issues/1897

Where a player cannot unequip a conjured item once equipped. The patch attempts to pick up on error conditions deriving from the ability to unequip, such as dropping the item, moving it into a container or attempting to sell it to a merchant. 
This should be done *after* the if (mDragAndDrop->mIsOnDragAndDrop) part, or dropping a legitimate item will not work when the cursor happens to be over a bound item.