The Button class is a simple button. Use TextButton for
buttons that should show text. This is the lightest weight
button used for non-text buttons like the arrow buttons
in a Scrollbar or NumericStepper.
The most common view for this button is CSSButtonView that
allows you to specify a backgroundImage in CSS that defines
the look of the button.
However, when used in ScrollBar and when composed in many
other components, it is more common to assign a custom view
to the button.