Replace non-linear response of camera stick with slow / fast modes

[rjmunro]

Normally the camera should only be able to move slowly even at full deflection. If you want it to go faster, you can hold the button on top.

Ideally, holding the button would accelerate to maximum speed over some configurable interval (e.g. 1s), and releasing gently slow it back down. This will mean you can give a quick tap to the button without any jump in movement speed if something is moving out of shot faster than slow mode can follow it.

Ideally, slow mode would be zoom sensitive, so a stick deflection angle would move the same % of screen widths / second at any zoom level. I.e. when you are zoomed in more, the camera would move slower.

(Expanded from description in #36)