Add Selection + Interactivity to Grid

[ecton]

On a quest to build a "listbox" type widget, I think the next step is to add a Dynamic<GridSelection> option to Grid.

• A grid selection should be able to be a single cell, a row, or a column. Maybe an "all" as well (open to feedback).
• A grid should have a setting as to whether clicking should do cell select, column select, or row select.
• Clicking on a cell should invoke a "cell clicked" callback. The callback should return EventHandling so that selection can be manually implemented.
  • If IGNORED is returned, the grid selection mode should be used to map the clicked cell to the active selection.
• Before cells are drawn, the highlight color should be drawn where the selected cells are located. If possible, a single rectangle should be drawn.

One last thing to consider is whether the Grid should associate a style component so that widgets can behave differently when they are in a selected cell in a grid. On one hand, it sounds useful. On the other hand, it sounds like a little too specific of a behavior to be encouraging widgets to perform with each other.