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/kitty/fix-paths.patch

@@ -0,0 +1,16 @@
+--- a/docs/Makefile
++++ b/docs/Makefile
+@@ -3,7 +3,7 @@
+# Patching is needed here for the following reason:
+# * importing the `constants` package from Kitty has a side effect that it
+#   creates the user configuration directory. This package gets imported
+#   while sphinx scans the code for documentation strings.
+#
+ 
+ # You can set these variables from the command line.
+ SPHINXOPTS    = -j auto -T $(FAIL_WARN)
+-SPHINXBUILD   = sphinx-build
++SPHINXBUILD   = PYTHONPATH=${PYTHONPATH}:.. HOME=${TMPDIR}/kitty-build-home sphinx-build
+ SPHINXPROJ    = kitty
+ SOURCEDIR     = .
+ BUILDDIR      = _build