Make "bokeh finished rendering heuristic" work with non-plot examples

[mattpap]

Currently they time out, because "finished" event isn't ever triggered.

[bryevdv]

@mattpap @canavandl FYI there is a tilesloaded event in the GMap API:

https://developers.google.com/maps/documentation/javascript/events
https://stackoverflow.com/questions/8677261/google-map-v3-map-loaded-event

Although it also looks like there is are suggestions to trigger off idle

For ImageURL and TileRenderer I think we can definitely have the actual image render trigger code to set a condition variable, then as soon as all the conditions are met (however many tiles/images there are) a "done" for that renderer can be generated.

cc @jbednar