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/iterm2/disable_updates.patch

@@ -0,0 +1,11 @@
+--- iTerm2/sources/iTermPreferences.m   2016-06-23 16:55:28.000000000 +0200
++++ iTerm2/sources/iTermPreferences.m   2016-06-23 16:55:42.000000000 +0200
+@@ -189,7 +189,7 @@
+                   kPreferenceKeyInstantReplayMemoryMegabytes: @4,
+                   kPreferenceKeySavePasteAndCommandHistory: @NO,
+                   kPreferenceKeyAddBonjourHostsToProfiles: @NO,
+-                  kPreferenceKeyCheckForUpdatesAutomatically: @YES,
++                  kPreferenceKeyCheckForUpdatesAutomatically: @NO,
+                   kPreferenceKeyCheckForTestReleases: @NO,
+                   kPreferenceKeyLoadPrefsFromCustomFolder: @NO,
+                   kPreferenceKeyNeverRemindPrefsChangesLostForFileHaveSelection: @NO,