Initial Gamepad support

Adds initial ingame gamepad support to minetest.

Full Formspec support is not implemented yet and
can be added by a later change.

src/client/keys.h

@@ -67,6 +67,12 @@ public:
 
 		DEBUG_STACKS,
 
+		// joystick specific keys
+		MOUSE_L,
+		MOUSE_R,
+		SCROLL_UP,
+		SCROLL_DOWN,
+
 		// Fake keycode for array size and internal checks
 		INTERNAL_ENUM_COUNT