Plan: Fence definition usability improvements

I've found some issues with MP's polygon fence support in Beta MP (and perhaps stable).

1. After uploading "FENCE_POLYGON_VERTEX_INCLUSION" commands, they are switching back to "FENCE_CIRCLE_EXCLUSION" when the "Read" button is pressed. I think this is happening because the Points column doesn't match the number of points in the polygon.
2. There's no way to completely clear all fence points
3. I think we should change the default Command type to be Polygon Inclusion instead of "FENCE_CIRCLE_EXCLUSION"
4. While we're at it let's rename the entries:

- FENCE_RETURN_POINT should be just "Return Point (Plane only)"
- FENCE_POLYGON_VERTEX_INCLUSION should be "Polygon Inclusion"
- FENCE_POLYGON_VERTEX_EXCLUSION should be "Polygon Exclusion"
- FENCE_CIRCLE_EXCLUSION should be "Circle Exclusion"

Start by tracing Mission Planner's polygon fence support and the Read-button handling described in the report. Reproduce the command/point mismatch and clearing behavior, then verify the default command and displayed names; done means all four listed usability changes work without changing valid fence types.

Written by the indexing model from the issue text.