Slider: make arrow buttons faster

[bill]

Very slow behaviour of the carrot/scrubber when B/F arrows are clicked on to move the carrot along the bar.

Taken from IBM Design Leadership recommendations.

[bill]

IIRC I talked to Doug about this and he was concerned about breaking the ability to set a slider value exactly using the arrows. For example, if clicking the arrow incremented the value by 10, there's no way to set the value to 7.

Perhaps we can just speed up what happens when you hold down the arrow button or arrow key(typematic), or maybe this is a wontfix.

[doughays]

(In [14282]) Fixes #6465. Added typematic movement for slider when clicking the buttons.