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
more work to finish merge
SPARQL10, SPARQL11 and exhaustive methods

This commit refactures the dropdown widget somewhat. In the rdfauthor.config.js,
the dropdown gets two additional keys, namely 'method' and 'use_exhaustive'.
The value for 'method' can either be 'sparql10' or 'sparql11', the value
for 'use_exhautive' is a boolean value.

'sparql10' will use a (rather slow) SPARQL 1.0 query which can only query
collections/dropdowns up to a fixed length. This is the default value, if
no value was found.
'sparql11' will use a SPARQL 1.1 query which is more flexible and can query
collections/dropdowns of arbitrary length and is also faster (but the query
uses property paths which are not supported by the vanilla OntoWiki/Erfurt).

The triples that (collectively) trigger a dropdown widget for ?PROPERTY are:

    ?PROPERTY rdfs:range ?CLASS .
    ?CLASS owl:oneOf ( ?RES1 ?RES2 ?RES3 ... ) .

'use_exhaustive' will (independently of the method) query all values of the
range class ?RANGE. To trigger this behavious, there must be triples of the
form:

    ?PROPERTY rdfs:range ?RANGE .
    ?RANGE sysOnt:displayAs "dropdown" .
    ?RES1 a ?RANGE .
    ?RES2 a ?RANGE .
    ...

Note: 'use_exhaustive' is a quick method and it also circumvents the problems
OntoWiki has with blank nodes.

There might be a better way to express the 'closedness' of a class using owl
constructs, so this might be changed. If the key 'use_exhaustive' is set to
false or not set at all, this behaviour will never be used.
Add support for ordering when using exhaustive
Merge remote-tracking branch 'origin/feature/rdfauthor-FormBuilder-dropDownTestWithSPARQL11' into develop

Conflicts:
	src/rdfauthor.js