[ADD] add embedded BaseX as data source

The following pull requests adds the possibility to add embedded BaseX as a new data source for the runner. We are quite interested in not using XQJ to communicate with BaseX, for the following reasons:

 - we are limited to XQJ for the communication, i.e. future enhancements like debugging are incompatible with XGJ
 - by not using XQJ a user does not have to start an external BaseX server
 - XQJ is closed source and we do love open source!

We are looking into further enhancements (in general and with specific focus of BaseX) for this plugin, this is a first start. We are also interested in more input also from other vendors, so we don't want to break anything. In fact, I think it would also be interesting for others (e.g. exist-db) to use an embedded version and it should be quite easy to add this now.
Sorry about the not quite optimal pull request, I am very new to IntelliJ and IntelliJ plugin programming (and git wasn't really doing what I wanted it to do...). We are actually very happy about comments and improvements/suggestions/hints.