Mouse camera control

[ryanshow]

The camera should be guided on the edge of an invisible sphere, with the center of the sphere being the object that the camera is looking at.

• Holding the middle mouse button should enable the user to drag the camera along the surface of the sphere. The mouse moving in the X direction should move the camera along the longitude of the sphere, and the mouse moving in the Y direction should move the camera along the latitude of the sphere
• The mouse wheel should make the sphere larger or smaller
• Holding shift + the middle mouse button should move the camera on a plane tangental to the sphere at the point the camera is at.

[ryanshow]

The first two camera movement techniques have been implemented.