Add UI control for "zoom"/window size in continuous monitoring mode

[breznak]

appConfig.WINDOW_SIZE or windowing.size effectively controls "size"/zoom of the graph presented with each update.

We can zoom interactively with mouse, but in streaming mode this does not work. The zooms freezes the graph and no updates are shown until zoom-out.

We should add a UI control for
https://github.com/nupic-community/nupic.visualizations/blob/master/client/src/app/appCtrl.js#L16
(a slider, numeric entry, ...) so users can set this "zoom" - limiting how much detail will they see as the graph is updating in monitoring mode.