Add scrollbars to viewport

[fpesari]

Hello,

along with #21 , the lack of scrollbars in the plot viewport make navigation very hard. It would be ideal to have a mini-map like they do in some text editors like KDE's Kate:

But short of that, I think that even simple scrollbars would make navigation much easier.

Thank you for listening, have a nice day.

[fpesari]

I noticed that #20 does implement zoom and pan, is my problem the browser I am using (Mozilla Firefox), is flowrun.io/scratchpad not up-to-date or are those functions only available to touchscreen/trackpad users?

[fpesari]

Linked it to #21 because the change I proposed there (a "Reset view" button, basically, which zooms out and centers the plot) would make this issue less critical.