Add support for DataTypeProperties

[JohannesLipp]

We need support for datatype properties, which allow the user to add simple datatypes to classes.

ObjectProperty (already in place) = Links one class (node) to another. DatatypeProperty (not existing yet) = Adds a list of simple datatypes to a class.

1. Please add support for the XSD built-in datatypes, from which the user could choose from a dropdown field with autosuggest as depicted in this screenshot:

   Please include the all built-in types from "anyType" down to "unsignedBte/IDREFS", but only skip the white "all complex types" for now.

2. Please update the export serialization by the following.

   • Add these new things to the export via owl:DatatypeProperty instead of owl:ObjectProperty
   • Set the rdfs:range to the datatype the user chose, i.e., https://www.w3.org/TR/xmlschema-2/#decimal etc.

• [x] fixes enise branch styling break up on graph view

[JohannesLipp]

• Change "Add Properties" label in sidebar to "Add Property"
• Hide the current input fields for property creation, and show them on button click (2 buttons: "Object Property" and "Simple Data Property"
• Visualize DatatypeProperties in the graph (e.g., as WebVowl does)
• We enable editing via onClickListeners on these edges (different behavior: dropdown menu for "target" chooses from pre-defined datatypes instead of existing classes in the vocabulary)

[JohannesLipp]

• Please also update the export to include owl:DatatypeProperty for these
• Please update the Property schema definition in the readme

[alihussainiam]

New issue have arisen which needs to be addressed separately that is:

• listener for normal node clicks allows adding props to datatype itself