Add a zoomslider control.
Simplify example and let it use real resolutions.

Thanks to @twpayne for the suggestion.
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.
Don't use non-exported method in example.
Remove CSS workarounds as suggested by @fredj.
Adjust test syntax.
Merge pull request #345 from marcjansen/zoomslider-own-ui-2

Add a zoomslider control. (r=@twpayne,@elemoine,@fredj)