Extract value relations

[tomasMizera]

This PR extracts ValueRelationFeaturesModel from previous FeaturesListModel that contained overcomplicated logic mixing logic for BrowseData and ValueRelations in one class. FeaturesListModel is also a base class for RelationFeaturesModel and RelationReferenceFeaturesModel as it can be seen on image below.

ValueRelation-, Relation- and RelationReference- FeaturesModels are used in respective form editors to list features from desired layers. They override several methods from base class like methods that set up feature request, title field or general setup method where configs are parsed.

Resolves #1640

• add test for ValueRelationFeaturesModel
• add test for FeaturesModel

[PeterPetrik]

just a brief read. 👏

[tomasMizera]

Please do not merge, I found one issue that needs to be fixed! Value relation fields are sometimes losing text (when opened in readonly mode)