gimp: add gimp-with-plugin derivation

By default all plugins from pkgs.gimpPlugins set are enabled.

Default location of plugins changed from $out/${gimp.name} to
$out/lib/gimp/${majorVersion}. Resulting derivation for gimp+plugins
is set as search path for plugins by default (additional tweaking in
gimprc done for old plugin scheme should be removed)

pkgs/applications/graphics/gimp/plugins/default.nix

@@ -6,9 +6,7 @@
 { pkgs, gimp }:
 let
   inherit (pkgs) stdenv fetchurl pkgconfig glib;
-  targetPluginDir = "$out/${gimp.name}-plugins";
-  targetScriptDir = "$out/${gimp.name}-scripts";
-  prefix = "plugin-gimp-";
+  inherit (gimp) targetPluginDir targetScriptDir;
 
   pluginDerivation = a: stdenv.mkDerivation ({
     prePhases = "extraLib";