Allow moving (instead of copying) files into Zotero

Cmd-drag on OS X, Shift on Windows and Linux

As requested in
https://forums.zotero.org/discussion/31717/delete-source-file-on-import/

Unfortunately due to https://bugzilla.mozilla.org/show_bug.cgi?id=911918
(and another issue on Windows) we can't show proper cursor feedback for
external file drags.