Improved whitespace handling

1) Rather than having the number of spaces set to `4` when *Insert spaces instead of tabs* is selected, allow the number of spaces to be configured through preferences.
2) Add a preference (and supporting implementation) that automatically removes trailing whitespace.

Start by locating the preference handling and the existing implementation for inserting spaces instead of tabs. Then trace how whitespace is processed so both the configurable space count and automatic trailing-whitespace removal can be implemented. Done means both behaviors are available through preferences and work as requested.

Written by the indexing model from the issue text.