Show progress indicator while loading tiles

[forman]

Often, loading tiles from xcube server take several seconds due to slow network connection or because dynamic tile computation by xcube server ist just time consuming.

It is currently not possible to see whether (1) tile fetching is still ongoing and (2) how many tiles are still to be loaded.

While loading dataset tiles, a progress indicator should be shown either as a map overlay or in the app's control bar.

See https://openlayers.org/en/latest/examples/tile-load-events.html