Create a new command file.openDroppedFiles so that we can support folder drop on Brackets.

This pull request is to consolidate different code that handle the drag and drop of files and folders by creating a new command for openDroppedFiles in DragAndDrop.js and calling it for all drag-and-drop triggered from the shell code. So it is part of the fix for issue #5015. There are two more pull requests from brackets-shell to complement this.