Use MonolingualTextValue instead of String for all terms

This was discussed to be the cleaner solution, though it creates
some overhead in terms of data size.

(The commit only contains some minor renamings for consistency)
Add StatementGroups

These groups of statements will be used to represent the groups of
Statements seen in the WIkibase UI. They will replace the list of
Statements in the current ItemDocuments.
Merge branch 'datamodel-general-update' of
https://github.com/Wikidata/Wikidata-Toolkit.git into
datamodel-general-update

Conflicts:
	wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/interfaces/DataObjectFactory.java
	wdtk-datamodel/src/test/java/org/wikidata/wdtk/datamodel/implementation/DataObjectFactoryImplTest.java