Fix breadcrumb to reinit dnd after ajax nav #5064

Now correctly reinitializing the breadcrumb drop zone after ajax
navigation.

This also fixes dropping onto the "files" app icon.
Note that dragging onto the "files' app icon while being in the root dir doesn't do anything.

I've tested it in Firefox 24 , Chrome 30 and IE8.

Please review @DeepDiver1975 @karlitschek @schiesbn