A directory-category for terminal emulators

This is a mostly cosmetical commit, in the sense it doesn't change the contents
of any package, but reorganizes the overall Nixpkgs expressions.

Terminal emulators are an ubiquitous tool for any Unix user; even the beginners
are routinely familiarized to it. And, manifestly, there are many
implementations of terminal emulators out there, from those traditionally made
in C and C++ to those written in Haskell and Go.

Terminal emulators deserve more highlight. This commit does that by creating a
category for them.

pkgs/applications/terminal-emulators/termonad/default.nix

@@ -0,0 +1,19 @@
+{ stdenv, ghcWithPackages, makeWrapper, packages ? (pkgSet: []) }:
+
+let
+  termonadEnv = ghcWithPackages (self: [ self.termonad ] ++ packages self);
+in stdenv.mkDerivation {
+  name = "termonad-with-packages-${termonadEnv.version}";
+
+  nativeBuildInputs = [ makeWrapper ];
+
+  buildCommand = ''
+    mkdir -p $out/bin $out/share
+    makeWrapper ${termonadEnv}/bin/termonad $out/bin/termonad \
+      --set NIX_GHC "${termonadEnv}/bin/ghc"
+  '';
+
+  # trivial derivation
+  preferLocalBuild = true;
+  allowSubstitutes = false;
+}