Add horizontal scrolling through the graph

[fedarko]

Via a scrollbar at the bottom of the screen. There are some good JQuery options for this; I don't particularly want to add JQuery to this project just for one feature, but if I can't find any suitable alternatives it should be alright.

We might want to consider having a vertical scrollbar, too, for certain connected components (e.g. RF_oriented_1.xdot, at least right now) that can't be visualized as a linear or even semi-linear path through the graph. Also, we should probably take graph orientation into account for this, since we'll obviously have more of a need for vertical scrolling for down-up/up-down rotated graphs. And I'm not sure if having both a vertical and horizontal scrollbar is the best idea—how much overhead would it have when added on top of Cytoscape.js? Arguably the major bottleneck in interacting with large graphs in Cytoscape.js is the delay inherent to interacting with nodes/edges/node groups: collapsing an individual cluster in a large graph might take 5 seconds, 3-4 of which are just Cytoscape.js waiting to register a "cxttap" event with that cluster's element.

This seems like a fairly minor feature (that will probably be nontrivial to implement), so I'll probably work on getting some more critical things done first.

[fedarko]

Utilizing a bit of custom CSS to define something looking scrollbar-ish (we could even go with just arrows for the time being) + something like on click -> cy.panBy() would make this pretty simple. Will do that.

[fedarko]

This issue was moved to marbl/MetagenomeScope#3