Add blueman apapters config to Xfce settings dialog (xfce branch ONLY)

This branch implements *only* xfce settings dialog support for blueman adapter configuration. 

Since the "thunar" branch conflicts with this, I'm also going to do a "thunar+xfce" branch pull request, which includes the commits from both branches merged together without the conflicts. Assuming both sets of changes are wanted, take that one instead.