This implements the keyboard shortcut handling and action invocation in order to provide a place in the infrastructure for keyboard events to trigger actions. This will allow binding of key events to actions like "move the focus to the next widget" and "activate button".
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.
