Fixes and cleanup to list related keywords (#989)

* Cleanup list keyword code and docs.

Small changes to functionality too:
- Get List Items optional argument renamed from `value` to `values`.
  This fixes #722.
- List Selection Should Be doesn't anymore allow mixing labels and values.
  Possible backwards incompatible change.
- Failure message from List Selection Should Be and List Should Have No
  Selection show both labels and values.

* Get Selected List Values/Labels return empty list when no selections.

Earlier this caused an error. Fixes #977.

* Deprecated "(Un)select From List". Fixes #988.

* Add "Unselect All From List". Fixes #987.