WIP refactoring:
* Replicate Tab_* class structure under Question as under AnswerDomain
* Debug-tab in Question properties form to show form defaults

Separate to/fromArray() database <-> object mappings from 
setDefaults/getValues() form <-> object mappings:
* Work with setDefaults() to fill forms (converse getValues to be done still)
* moved (part of) toArray() code
* still with explicit $this->Translation->toArray(); in several models

Extended WebEnq4_Form:
* Provide default isValid and isCancelled functions that are repeated now
* Adding tooltips for fields in the AnswerDomain_Item table

Added "missing" as attribute of AnswerDomain
merge with changes Jaap-Andre
Polishing question forms and finishing refactoring
Removed debug tab
WIP: CSS changes to distinguish 'tabbed forms' and regular forms
Merge commit with 'origin/master'