Exclusive zone widget

[darkwater]

There should be a widget to represent the exclusive zone, eg. for fullscreen overlays.

My current implementation is a widget that listens to its laid out height, and updates the exclusive zone appropriately. Some ways we could improve this:

• Add an exclusiveEdge parameter to also manage the exclusive edge
• Throw an exception if two ExclusiveZones are present, because the wayland protocol only allows for one
• Be able to parent to a Stack to automatically lay out correctly, eg.:

Stack(
  children: [
    ExclusiveZone(
      edge: ShellEdge.bottom,
      child: SizedBox(
        height: 64,
        child: ...
      ),
    ),
  ],
)