Fix Slider increments on keypress

Instead of always adding/subtracting 1/10 of the range use the step size when
the arrow keys are pressed. In addition add accessible actions to inc/dec the value.

Task-number: QTBUG-39099
Change-Id: I4fdf2adc8912ab1fa8838be56af4e798eeb59781
Reviewed-by: Caroline Chao <caroline.chao@digia.com>