something like a minimap

[nilsherzig]

Is your feature request related to a problem? Please describe.
I often find myself working on a small part of a diagram / flowchart or something like that, and having to switch between the "edit" view and the "overview" view all the time, which takes time and breaks my flow.

Describe the solution you'd like
Something like a small minimap which shows the entire or a selected part of the document the user is currently working on.

[Doublonmousse]

I wonder if this should be linked to #639 as well (would make sense to click on the minimap to explore the document for the infinite modes).

Maybe we can do it okular-style for the vertical page mode ?