Allow synchronisation between the two Markdown editor windows

Currently the two markdown editor windows (text editor and preview) are fully independent.

It would be cool to have some synchronisation between them, for example scroll the rendered window when the plain text is scrolled. It would also be nice to click a word in the rendered view and jump to the corresponding position in the plain text view.

As this might be complicated, a first step would be syncronisation on headlines #, ##, ### in combination with a plain text comparison.

But maybe its also possible to add back-references to the paragraphs in the render view.

Probably the Markdown language server (QTCREATORBUG-29755) can also be helpful.