External file drag and drop (#3933)

Deals with https://github.com/zed-industries/community/issues/1317
Deals with https://github.com/zed-industries/community/issues/486

Reworks pane drag and drop code to support 
* dropping external files into main pane (supports splits same as tabs
and project entries drop) — this will open the file dropped
* dropping external files, tabs and project entries drop into the
terminal — this will add file abs path into the terminal

Release Notes:

- Added a way to drag and drop external files into Zed main & terminal
panes; support tabs and project entries drop into terminal pane