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/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(client_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/clientlauncher.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/tile.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/joystick_controller.cpp
 	PARENT_SCOPE
 )