Move keys to a separate mod (Fix #1490)

mods/keys/README.txt

@@ -0,0 +1,21 @@
+Minetest Game mod: keys
+==========================
+See license.txt for license information.
+
+Authors of source code
+----------------------
+Originally by celeron55, Perttu Ahola <celeron55@gmail.com> (LGPLv2.1+)
+Various Minetest developers and contributors (LGPLv2.1+)
+
+Authors of media (textures, sounds, models and schematics)
+----------------------------------------------------------
+
+Textures
+--------
+Gambit (CC BY-SA 3.0):
+  keys_key.png
+  keys_key_skeleton.png
+
+Features
+--------
+This mod uses the key API as defined in game_api.txt section [Key API].