Introduce event types for key, mouse, joystick release (UP)

* Button: Change mouse button event to released state; this matches
the behavior of Cocoa, Win32 and most any other GUI.

* BitmapButton: Change mouse button event to released state; this
matches the behavior of Cocoa, Win32 and most any other GUI.

* examples/gui: Remove WINDOW_MOUSE_DOWN event listeners for button
examples.