Middle Mouse Scrolling

-Makes camera scrollable via middle mouse button, moves independent of edit mode in LevelEditor.
-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.