Improve usability of both-view

**Is your feature request related to a problem? Please describe.**
Kind of, scrolling with "scroll-sync" turned on gets laggy pretty fast. Especially for big documents.

**Describe the solution you'd like**
Currently you can hover over the edge between the two panes to unlink "scroll-sync". This fixes the scrolling being laggy but removes quite a big part of easy-of-use, especially in big documents. Maybe you can implement a third option where both panes get synced whenever you click on the page/editing pane/move focus from one pane to another/etc.

A good simple implementation would probably consist of only syncing the view pane to your editing location whenever you enter a new character, and when pressing a special shortcut you sync the editing location to the view location. Really the world's your oyster, the possibilities are endless and I'll be looking forward to something like this!