Add ability to change replay playback speed via arrow up/down

[Theighlin]

I've heard from someone used to cutting edge that watching replays in lazer is not as good because there are no keybinds to change replay speed (jumping with the arrows is not a substitute as you still can't grasp what happened clearly if the speed doesn't change).
The only way for now is to approximately drag a slider on an overlay that autocloses.

I propose changing speed in increments of 0.1x and if possible of 0.05x with a modifier key like ctrl.
Maybe also adding number keys to change speed could be good, or at least 1 key to restore to 1x speed.