Use OSM resolutions as fallback resolutions.

Hat tip to @twpayne for suggesting this change in the defaults.
Add an assertion to the dragchange handler.

This ensures that the result of resolution calculation stays inside of the
allowed bounds.
Unregister previous event listeners, if any.

Thanks for spotting this, @twpayne.
More specific argument type of callback method.
Correct API documentation.
Use getContentBoxSize() for slider initialisation.

Initial note to revisit this came from @twpayne; thanks.