Add the possibility to specify plugin set to
be used as overridable `thunar` derivation argument.
New nixos config attribute:
`services.xserver.desktopManager.xfce.thunarPlugins`
that allows user to specify plugins in the context
of nixos.
Tests:
- With and without plugins.
- Using the nixos attributes.
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).
