FilterUtil Category Plugin, fixes and documentation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| Refs tickets  | #1205
| License       | MIT
| Doc PR        | 

Reimplemented the FilterUtil category plugin and done som mine fixes. Rewrote the developer documentation.