thunar: plugin improvements (close #9908)

Improve thunar/thunarx so that thunar's plugin directory
could be overriden using a `THUNARX_MODULE_DIR`
environment variable so that plugins not built along
with the `thunar` package could be installed as well.

This could allow one to wrap `thunar` in a `thunar-with-plugins`
package where plugins are declared instead of being found (a
bit in the same fashion as gstreamer plugins).