Removed dropdown widgets return value null

When a widget gets removed (via the 'minus' button) it is in fact still
active and hidden. When submitting a change, RDFauthor tries to fetch a
value even from the removed widgets. Usually they return null, if there is
no value. In case of the dropdown widgets, there was an error which is fixed
in this commit.
Slight refactoring in preparation of more query options
Some refactoring to prepare wider options

The values for dropdown fields can be queried with SPARQL1.0 or SPARQL1.1
In future, there should be an option to choose which method will be used.
This commit prepares this refactoring.
Merge branch 'feature/rdfauthor-FormBuilder-dropDownTest' into feature/rdfauthor-FormBuilder-dropDownTestWithSPARQL11-merge

Conflicts:
	src/widget.dropdown.js
More merging work