(#1191) Disallow picking up if inventory disabled

Quickly fixes Bug #1191 by disallowing any sort of drag and drop if the window manager hasn't yet enabled the inventory.
This does not replicate the Vanilla behaviour completely:
* Once inventory is enabled you may still drag and drop the release papers into your inventory
* If you attempted to drag and drop the papers, leaving the menu will not activate the papers

I am willing to implement these missing behaviours, if it is needed. I would, however require a little advice as to how to do it.