thunar: improvements (close #10306)

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.