Xfdesktop: move should be default instead of copy

[Gregoire Gentil]

Brian,

Perhaps, it would make sense to default a move instead of a copy when a
drag and drop action is triggered by the user. Indeed, Thunar has a
default move action when drag and dropping. Moreover, if you drag and
drop a file to the trash, it doesn't make a lot of sense to see the plus
overlay icon.

Thanks,

Grégoire