Make joystick activation optional, so that Irrlicht doesn't consume all joystick events.  This allows us to pass joystick enumeration data to the user app.  Revert example 04 to just do key movement, but have it show framerate independent movement.  Add a new 19.MouseAndJoystick example.  Tested with Win32 and Linux devices, and with the SDL device on Windows and Linux.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1717 dfc29bdd-3216-0410-991c-e03cc46cb475