Support IntelliJ 2019

[fhoeben]

The plugin works with IntelliJ 2019, but building with 2019 as dependency fails. Some classes have moved to separate .jar files and others seem to have been removed completely.

I was unable to find mention of the removed classes in the documentation, maybe they have been removed for quite some time?

This will hinder development of a new version should it be necessary...

My current (not compiling) efforts to get IntelliJ 2019 working can be found in https://github.com/fhoeben/idea-fitnesse/tree/intellij19