Complex fence support issues (affecting Copter and Rover)

This is a list of known issues with the MP's support of Complex Fence support that was introduced with ArduPilot Copter/Rover 4.0. This issue was prompted by at least one user running into troubles with the feature [here on discuss](https://discuss.ardupilot.org/t/copter-4-0-0-released/50545/48).

1. "There was an unexpected error" message when attempting to upload a polygon fence. This was a very simple fence as shown below:

2. buttons and messages do not differentiate between missions and fence points:

- "Read WPs" should be renamed to be more generic or should change to "Read Points" (or something similar) when "FENCE" drop-down at top is selected
- "This will clear your existing planned mission, Continue?" should be "This will clear your existing fences, Continue?"

3. We should remove the two step-process involving the polygon drawing tool menu on the top left. The current process is:

- select Fence from the drop-down on the right
- click on the polygon icon on the top left, select draw a polygon
- draw a polygon on the map
- click on the polygon icon (again), select "fence inclusion" or "fence exclusion"

Instead the user should just select "fence inclusion" or "fence exclusion" right at the beginning instead of first having to select "draw polygon".
Also the little polygon icon should go green when the user is entering a polygon and if they click it again it should probably have the option to abandon the fence they're drawing.
An alternative method more consistent with the way that waypoints are added would be to use the equivalent of the "Add Below" button. Either have four buttons for "Add Polygon Stay-in", "Add polygon stay-out", "add circle stay-in", "add circle stay-out" (or whatever names you want) that then trigger slightly different drawing methods.

4. "There was an unexpected error" when writing a blank list of fence "commands". If the user wants to clear all the fences, they would intuitively clear the list and press write (just like they do for WP). this does clear the list of fences but also produces this error.

**Fixed issues:**

- [x] button below slider looks like a Japanese yen sign and it's help text is "Add a line to the grid bellow" which is incorrect (also "below" is spelled with a single "l").
- [x] no way to upload circular fences

Start from Mission Planner's Fence drop-down, polygon drawing tool, and read/write controls described in the issue. Reproduce polygon upload, blank-fence writing, and the fence-specific labeling and drawing flows. Done means the listed errors are resolved and fence controls and messages clearly distinguish fences from missions.

Written by the indexing model from the issue text.