Changing speed in Spectator mode cannot be rebound

The bug

It is not possible to rebind the functionality that permits changing speed in Spectator mode while scrolling.

Fixing this would probably require allowing controls to be bound to scroll wheel movement (MC-268284).

How to fix

Add two new entries to Key Binds: one for increasing speed in Spectator and one for decreasing speed. One would be bound to scrolling upwards by default, the other to scrolling downwards.