Allow adding of reference and show position in text area

[JSAbrahams]

Use RichTextFX text area for more advanced features, including highlighting of references. A few features:

• On hover, we can see what was referenced where (both as a tooltip and highlighted in the overview)
• We can add references
• References are removed if the text is removed

A few things:

• Text positions of references are not updated if we add new text
• We cannot remove references without deleting all text starting at its starting position

TODO

• [x] implements #10
• [x] implements #5
• [x] Allow adding of new citations, using the global citation view