v0.5.1

A hovered row, and a tree that says which rows it drew.

tree now takes the same onRow(node, index, y) the table has had, so a click on a scrolled tree can be turned back into the node under it. Table, list and tree take hovered, a row drawn a shade lighter than the background that always loses to the selection, and every scrollable widget takes onHoverRow to feed it. The widget only hears the pointer while it is inside, so the book shows the two-line mouse listener that clears the hover when a move goes unclaimed.

The tree gained the scrollbar the list and table already had, and the headless test screen gained hover(x, y).

Nothing renders differently unless hovered is set, so the port fixtures do not move.