Feature Statistics: Rename to Variable Statistics

[janezd]

Issue

Closes #7084.

Description of changes

• Rename Feature Statistics to Variable Statistics.
• Rename the column from "Name" to "Variable" in the Variable Statistics table.
• Rename the output variable from "Feature" to "Variable".

Backward compatibility (non-)issues:

• Renaming the output variable will break existing workflows that ... huh, that ... filtered by this variable?! Such workflows are unlikely, and the fix would introduce some ugly code, so let us not worry about it.
• Since only the name (and not the qualified name) has changed, existing workflows won't be affected.
• I added a keyword so that typing "feature" still finds the widget.

To do:

• Change documentation ... after the change is confirmed and I (re-)discover how to properly clone the new repo. :)

Includes

• Code changes
• Documentation

[markotoplak]

Column Statistics?

[wvdvegte]

Column Statistics would also be better than Feature Statistics