Factor out AccessibleValueHandlerKeyboardHelpSection for UI components that use AccessibleValueHandler trait.

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Class hierachy for keyboard-help sections currently looks like this:

class SpinnerControlsKeyboardHelpSection extends SliderControlsKeyboardHelpSection
class SliderControlsKeyboardHelpSection extends KeyboardHelpSection

For someone making changes related to Slider, it's not going to be obvious that they are affecting Spinner.

Something like this would be safer and more consistent with the relationship between Spinner and Slider:

class SpinnerControlsKeyboardHelpSection extends AccessibleValueHandlerKeyboardHelpSection
class SliderControlsKeyboardHelpSection extends AccessibleValueHandlerKeyboardHelpSection
class AccessibleValueHandlerKeyboardHelpSection extends KeyboardHelpSection

AccessibleValueHandlerKeyboardHelpSection would likely be a base class that is not intended for instantiation, so protected constructor.