Add support for kinetic effect while dragging.

Only dragpan interaction is supported for now.

The kinetic effect use the maths below (thx @twpayne):

Rate of decay (number, must be negative) = α
Minimum velocity (pixels/millisecond) = v_min
Initial velocity (pixels/millisecond) = v₀

Velocity at time t (pixels/millisecond) =
    α⋅t
v₀⋅ℯ

Distance travelled at time t (pixels) =
   ⎛ α⋅t    ⎞
v₀⋅⎝ℯ    - 1⎠
─────────────
      α

Duration of animation (milliseconds) =
   ⎛v_min⎞
log⎜─────⎟
   ⎝  v₀ ⎠
──────────
    α

Total distance travelled (pixels) =
-v₀ + v_min
───────────
     α

Easing function =
   ⎛ α⋅duration⋅t    ⎞
v₀⋅⎝ℯ             - 1⎠
──────────────────────
     -v₀ + v_min
Activate kinetic effect by default