Revert "Tune up previous drag-and-drop commit"

This reverts commit 157d7c8560d2aa9a9286317a0a4010734018f76f.
Revert "Fix file drag-and-drop behavior"

This reverts commit b3ca8a76ea868c3f1e8bc9fae9381f79ec824683.
New and improved fix for file drag-and-drop.

Handles files dragged-and-dropped on frames and Aquamacs dock icon, with behavior
appropriate to one-buffer-one-frame-mode status and frames in current space.