Allow dragging of entities from the inventory to the world

It should be possible to drag inventory item to the world, and when dropped there it should be possible to place them. I.e.
1) The user grabs an inventory item and drags it to the world/render are (i.e. where there's no gui)
2) The user released the mouse button and can now more precisely place the entity in the world. For this the same functionality as when moving entities should be used. Note that entity isn't actually placed in the world yet (just as entities aren't moved until the user pressed the left button).
3) The user presses the left mouse button, at which the entity is moved from the inventory and placed in the world. Alternatively the user presses escape and it's all aborted (the entity stays in the world).

As further phases in this blueprint:
* Allow picking up stuff my moving them to the inventory.