Add more "concrete" navigation

[fedarko]

Todd suggested I include:

• A navigation box (like in the bottom corner of Google Maps) to help users avoid getting lost in really large graphs. An extension for this exists already here.
• More clearly defined panning/zooming functionality. An extension for this exists already here.

So I should definitely add these extensions and see how they work for graphs, adjusting accordingly.

[fedarko]

Tried adding https://github.com/cytoscape/cytoscape.js-panzoom -- it works, but:

1. Zooming using the widget is kinda strange, since we don't have a min/max zoom set
2. Panning is really, really slow on large graphs, like component 1 of RF_oriented. Like, "basically unusable" slow at times.

For now I'm going to store the modified code on my computer and work on other problems.

[fedarko]

#120 provides a good opportunity to integrate something like this.

[fedarko]

This issue was moved to marbl/MetagenomeScope#7