sound: Make sliders more accessible with keyboard

The volume slider only has a page up/down increment of 1%, and can't be moved with the arrow keys, but still consumes arrow key presses with a beep sound.

Fix this by adding a step increment of 1%, and increasing the page increment to 10%, as that one should be bigger. Do the same for the subwoofer slider.

Furthermore, the balance slider has both a page and step increment that are way too big to use effectively.

So, set the page and step increments also to 10% and 1%, respectively. Stepping with arrow keys close to 0 requires rounding to avoid an additional micro step, so let's add that as well.