## Description
This makes the `focusNode` for `SelectableRegion` optional so that:
- Users of the widget are no longer required to use `SelectableRegion`
from within a `StatefulWidget`
- They aren't likely to forget to dispose of a node they didn't supply.
- Simpler to use, and the node is not used very often anyhow.
Also made the `SelectableRegion` sample actually use `SelectableRegion`.
## Tests
- Modified all the `SelectableRegion` tests to remove 3 identical lines
of boilerplate from each (except 2, which actually used their focus
nodes).
4d3bbf30c8