Any disk can now be thrown using mouse.
Some refactoring.

The code for getting the disk under the mouse pointer refactored into
a function.
Subtracting a vector from a point implemented.
Mouse UI changes.

 - Disks can be dragged and moved using the left button.
 - Disks can be thrown by being dragged using the right button.
 - The smaller disk can be put at any location by clicking the middle
   button on an empty space.