This adds support for an animated focusColor and hoverColor to InputDecorator. This color will blend with the background over a fade in period whenever the InputDecorator is focused or hovered, respectively.
It also adds a Listener to the TextField to listen for hover events.
This implements focus and hover handling for Material buttons. It inserts Focus widgets into the tree in order to allow buttons to be focusable via keyboard traversal (a.k.a. TAB traversal), and Listener widgets into the InkWell to allow the detection of hover states for widgets.
Addresses #11344, #1608, and #13264.