Dragging a URL over an item in the Zotero pane should create an attachment

[dstillman]

Dragging a URL onto an item in the item pane should create a child attachment

A dialog should pop up with two radio buttons, "Save link to page" and "Take snapshot of page"--probably link selected by default. (Maybe we can save state and always offer the last one used by default?)

[dstillman]

Same goes for dragging into the Library or a project, of course (independent files)

[dstillman]

This should probably do something for dragging to whitspace/colletions as well, though I'm not sure what--possibly a dialog with the option to create a new item from the page, create a link or create a snapshot (and/or a combination of the above).

But there's currently no way one-step way to create a child attachment without the Attachments tab open, so the main part of the bug addresses that--this part wouldn't really add any unique functionality and is mainly for consistency.

[dstillman]

(In [1186]) Closes #142, Dragging a URL over an item in the Zotero pane should create an attachment

Can drag links, images, or the favicon of the current page

Also allows dragging over collections and item pane whitespace to create standalone attachments, but if we want to allow that at all, it would probably be better to create new webpage items with snapshot items (like Create New Item from Current Page) instead.