Add ability to drag and drop palette items before existing items

Right now, even if you drag and drop before an existing item, the new palette item is always put at the end.

One consequence, is that if you're on small screen and have some existing item taking all your screen and you add a new item before it, you don't see any change on your screen and you think it didn't work.

Note: This was reported to me by a user and I'm creating this issue on his behalf.