Add drag and drop support for opening files.

**Note: this pull request depends on adobe/brackets-shell#272**

Easily open files by dragging and dropping onto Brackets.

If none of the dragged files can be opened, the drag is rejected. If some invalid files (or folders) are included, Brackets will open the files that can be opened, and show an alert for the files that couldn't be opened.

Drag and drop for folders will be in a follow-on pull request.