-Makes clicking & dragging the middle mouse button pan the camera in level-edit
 move, independent of edit mode.
-Added MouseCameraScroller to process input & handle scrolling the camera.
-Mouse button 2 up and down ignored in LevelEditorScreen
-Add a new MosueCameraScroller input processor to LevelEditorScreen's
 InputMultiplexer
-Scrolling is modal--when scrolling, other buttons won't work.