xfce4-14: move to xfce and have only one version of xfce

All code that was at xfce4-14 has been moved to xfce/*.
Old expressions that aren't rewritten might be abandoned or broken.

Additonally I've ported the xfce4-14 thunar expression to support
thunarPlugins. We can now support this interface in the Xfce module
again, although I'm not sure if we have any plugins packaged that support
latest thunar.

pkgs/desktops/xfce/thunar-plugins/archive/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchFromGitHub, pkgconfig, xfce4-dev-tools
-, gtk
-, thunar-bare
+, gtk2
+, thunar
 , exo, libxfce4util, libxfce4ui
 , xfconf, udev, libnotify, hicolor-icon-theme
 }:
@@ -21,8 +21,8 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [
     xfce4-dev-tools
-    thunar-bare
-    exo gtk libxfce4util libxfce4ui
+    thunar
+    exo gtk2 libxfce4util libxfce4ui
     xfconf udev libnotify hicolor-icon-theme
   ];