Create regular item rather than snapshot via link drag and drop

[dstillman]

1) If dragging a URL over an existing regular item, create a child snapshot.

2) If dragging a URL to a collection or the top level of the items pane, load the target page in the background and determine if there's a translator for it. If there is, save via the translator. If not, perform the equivalent of Create New Item from Current Page, with a child snapshot optionally created according to the standard snapshot pref.

​http://forums.zotero.org/discussion/5984/