Add (language specific) userdocumentation to the attribute-properties and to the entity-properties

Currently in a listview (and elsewhere) hovering over the column-title you get shown a tooltip displaying the exact same title.

This functionality can get increased in usefulness if the hover-text would get filled with extra information. This can be a few words or several lines of text. The only correct place for defining that extra information is in the properties of the entity model's attributes.

The idea is to add textfields to entities and to the entities' attributes. These text are UI-text, so they should be language specific.

These text should by standarad then get used for the on-hover tooltips.

NB. the current documentation field is not language specific and meant to be read by developers.