Codemirror blob view: Improve scroll position and line selection when document is loaded

Created by: fkling

The current implementation causes CodeMirror to retain the current scroll position (if possible) when switching between files which can be surprising. I think ideally we would remember the current scroll position for each file individually and restore it (within a single session of course) if there is no line selected in that document.

Related to that we current set and scroll to the selected lines after the document was loaded, in second transition. The implementation should be changed so that this happens at the same time as the new document is set.