Populate cache

When `#populate-cache` is included in the URL, display two new buttons.  The first one, "Populate", will request tiles for levels 0-5 for all data sources that are in categories that are open on the data panel.  The second one, "Pop All" will request tiles for levels 0-5 for all data sources, by opening all categories and waiting for them to load.

Status/progress information is logged to the console.