thunar-sendto support ONLY

This branch implements *only* thunar-sendto (plus the rename of the configure option for nautilus-sendto).
Since the "xfce" branch (separate pull request coming soon) to add bluetooth adapter configuration to the xfce setttings meu will conflict with this branch, 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.