Optionally install Thunar support for blueman-sendto

Suggested by GreenLunar in Issue #26
https://github.com/blueman-project/blueman/issues/26
Renamed --enable-sendto to --enable-nautilus-sendto

This clarifies that the option enable nautilus-sendto, as there
is also a thunar-sendto option now.
Add blueman apapters config to Xfce settings dialog

This partially addresses Issue #25

TODO: Figure out how to make this pluggable - here's what ochosi in
      the xfce devel channel said:

  <+ochosi> rworkman: anyway, this is what you mostly have to do
  (create a gtkplug and "plug it in"):
  http://git.xfce.org/xfce/xfce4-power-manager/tree/settings/xfpm-settings.c#n2192
  obviously, in the builder/glade file, the plug-child widget has
  to contain all that you want to be shown in the plug

This looks like it should be helpful, but I'm not competent enough:
http://python.developpez.com/cours/pygtktutorial/php/pygtken/sec-PlugsAndSockets.php