FocusTraversalPolicy keep the previously visited node to avoid hysteresis. But even if the visited focus has been disposed, FocusTraversalPolicy will still use it to requestFocus, which will cause FocusManger to get an abandoned node to get the focus.

Replacing the algorithm for finding the first focusable item in the focus tree. Somehow it was a kind of gibberish before, and really didn't work or make sense.

This just fixes up some comments for DefaultFocusTraversal, and fixes a minor bug when setting the onKey on a FocusNode on creation before attaching to it.

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.

This implements a DefaultFocusTraversal widget to describe the focus traversal policy for its children, defined by a FocusTraversalPolicy object from which custom policies may be created. Pre-defined policies include widget-order traversal, "reading order" traversal and directional traversal.