[RFR] Isolate field directives scope to make them testable

Pros: Testability, speed (allows GC)
Cons: no cons.

- [x] bootstrap directive unit tests
- [x] isolate field directives
- [ ] ~~isolate column directives~~ (o be done in a future PR)

I chose to merge directives template with directive code when the template is simple - this eases the development.

Two directives remain non isolated:
* Datagrid (requires a larger refactoring)
* Template (very generic, also used un the list and show view).