xfce.thunar-dropbox-plugin: init at 0.2.1

A thunar dropbox plugin that allows to copy links and
add files to dropbox.

Depends on new `xfce.thunarx-2-dev` plugin sdk package instead of
thunar. Doing so seem standard on other distributions such as
Ubuntu and narrows depedencies to only the sdk headers (i.e.:
what's needed by plugins).

Note that replacing the `xfce.thunarx-2-dev` dependency directly
by `xfce.thunar` work equally well. However it now would be
impossible for the `thunar` executable to depend on the plugin.

Tests:

 -  When dropbox daemon active, functionalities are added
    to right click on file and folders menu.
 -  When dropbox daemon inactive, functionalities **not** added
    to menu.
 -  Successfully copied a dropbox link.
 -  Successfully moved a file to dropbox.