Allow costum stores + make operators configurable in manifest.json

[njakuschona]

With this pull request we want to introduce the possibility to make costum stores queryable by the dn_querybuilder. Therfore we adress two main components.

• Handling a request promise without a total promise.
  • Therefore we proof in QueryController.js wether a total promise is attached. If this it not the cas it just waits until the primary promise resolves

• Allow to configure the availabe comparison parameters

  • Here it is possible to overwrite in the app.json or manifest.json the "operations" parameter.
    • parameters for the "default" can be overwritten or a new field type can be added
    • a new operator Class can be creatred. Here the field then must have an attribute with the matchin operator Class

  The documentation in the Readme is currently quite rudimentary, please comment if it should be extended