Add modified attributes for vars dictionary.

- new external commands
- raw values for 'vars'
- OnVarsChanged() signal for db_ido customvariablestatus updates
- refactor db_ido customvariable config updates

Refs #5956
Refs #5962
Update documentation for external commands.

Refs #5956
Refs #5609
Fix custom var override.

Refs #5956
Refs #5962
Merge branch 'feature/cv-mod-attr-5956' into next

Fixes #5956
Fixes #5962
Fixes #5609