Add thunar-sendto-blueman.desktop

Closes #26
Closes #149
Optionally install Thunar support for blueman-sendto

Closes #146
Renamed --enable-sendto to --enable-nautilus-sendto

This clarifies that the option enable nautilus-sendto, as there
is also a thunar-sendto option now.
Fixup a few issues with thunar-sendto support

Closes #151
Add blueman apapters config to Xfce settings dialog

This partially addresses Issue #25

TODO: Figure out if it's possible to make this pluggable
and if so, how to do that. Probably this involves including
something from libxfceui, but we'll see...