Allow folder drop on top of Brackets app.

This pull request requires two other pull requests -- https://github.com/adobe/brackets-shell/pull/353 and https://github.com/adobe/brackets/pull/5590. 

I've reviewed https://github.com/adobe/brackets-shell/pull/353 and ready to land it in master. But this and https://github.com/adobe/brackets-shell/pull/353 need to be landed together after landing https://github.com/adobe/brackets/pull/5590.