[Dev] Drag and drop import

[Jeffrey Harris]

Hi Folks,

I just checked in an experimental "drag an ics file into Chandler"
feature.  I developed the functionality on Windows, I haven't tested it
on Mac or Linux.  More testing would be welcome.

Here's what should happen:
- Normal item drag and drop should still work
- files dragged into the sidebar should be accepted anywhere (not just
over collections)
- ics files dragged onto the sidebar should import into a new collection
or whichever collection you dragged the file over
- If an ics file is dragged into the calendar view, it should import
into the currently selected sidebar collection

Sincerely,
Jeffrey