further style and logic on task form brings some changes (see details)

- grade-input and grade-uri get small style changes
- grade-dropwdown-menu gets total restyling
- grade-dropdown-menu gets also validators
- task validation in place, except for graph misalignment checks (to do)
- graphs can only be refresh under edit
- dropitems show also area (quick solution, maybe good enough)
- summary show invalid states (useful to detect misalignments)
- view-level mechanisms help dereferencing deep bean properties