auto-expand folders on hover

[linuxgod]

collapsable folders look nice in 0.2 beta.  however, there are two things which would be useful:

1. auto-expand a collapsed folder list when dragging a message over and hovering on the collapsed folder.

2. auto scroll up/down when dragging a message to the folder pane and trying to go above/below the viewable list (Eg. when your list of folders is longer than your browser screen)