* Need to be able to control whether TreeView uses the primary
  ScrollController or uses a custom scroll controller
* If TreeView has shrinkWrap set, then do not use the primary scroll
  controller

These changes are needed so that if a parent ListView contains a
TreeView and the ListView has a custom scroll controller, then we
can avoid using the primary scroll controller and avoid exceptions
like "The provided ScrollController is currently attached to more
than one ScrollPosition".

A variant of SingleChildScrollView that provides horizontal scrolling
via the mouse wheel.

This is needed due to changes in Flutter 2.5, where the mouse can no
longer be used to drag and scroll horizontally on web/desktop platforms.

https://docs.flutter.dev/release/breaking-changes/default-scroll-behavior-drag

A `CommandBar` control provides quick access to common tasks. This
could be application-level or page-level commands.

See https://docs.microsoft.com/en-us/windows/apps/design/controls/command-bar

The current implementation does not implement the open/closed state,
nor does it implement the secondary commands "more" button with dynamic
overflow of items into the secondary commands menu.

Also changed cross-axis alignment for items within a CommandBar to be
center-aligned. Needed so that items will be properly vertically aligned
(including separators) if items are of different heights.

* A CommandBar now takes a list of primary and secondary command bar
  items. The secondary command bar items do not yet appear anywhere.
* The primary command bar items are intended to be displayed on the
  horizontal area of the command bar.
* Conceptually similar to PaneItem, CommandBarItem is now given a
  "display mode" when building the widget. The display mode indicates
  if the item is being rendered on the primary (horizontal) area
  (and if it should display in "compact" mode) or the secondary
  (flyout menu when "more" button is pushed) area.
* A CommandBar can be given a `compcatBreakpointWidth` that indicates
  that if the widget is rendered with a smaller width, then it should
  render all of the items on the primary area in "compact" mode.
* Foundation is laid for supporting the secondary area and an overflow
  behavior mode of "dynamic", but is not yet implemented.

Similar to Wrap, but only displays widgets on a single run, and then
hides widgets that would have overflown. If widgets are hidden, then it
will display the specified "overflow widget".

This is useful to implement the dynamic overflow mode of command bars.

It could also be useful if a vertical nav bar wanted to implement a "more"
button instead of vertical scrolling.

Also add mainAxisAlignment and crossAxisAlignment fields

* Illustrates right-alignment and dynamic overflow modes.
* Moves command bar examples into a new navbar pane to avoid clutter.