Add a minimap feature

[leissler]

For many node-based tools, it is very important for users to be able to quickly pan along a graph and keep an overview of its structure. Although there is a "center viewport" button, it is far better supported by an interactive Minimap, which is not only by chance one of the most used navigation features in such tools. In large graphs, this is a lifesaver for everyday work.

[Doppelkeks]

Have seen such a thing in several graph tools. Would be a good addition, but I currently consider it "Nice to Have" and not integral. To keep an overview you can already use the "Frame" action in the toolbar or via shortcut. Might revisit at a later stage. So long, always happy about pull requests :)

[leissler]

This could be a good start: https://github.com/Unity-Technologies/UnityCsReference/blob/master/Modules/GraphViewEditor/Elements/MiniMap.cs

I just gave it a quick try, but it's not as simple as I thought. Maybe I can find the time for this.