Annotator needs to significantly speed up in long full-texts

[davidzbiral]

The Acta S. Officii (ca. 200k words) shows significant limits of Annotator - scrolling (whether with mouse wheel, PgDn, or vertical bar) is slow and unreliable, the screen does not update fast enough, the selection operations only are done after 10 seconds, which prevents to do them precisely (see #2488, #2196 - marked done, but in longer documents the problem persists). We need the app to work more efficiently in these cases, some lazy loading or something so that it does not need to bear the full weight, etc. Text editors (Sublime, GDocs, Notepad, MS Word) are able to do that, so we must be able too.