Add new map.setFocus function

**work in progress**

@twpayne is that what you had in mind?

`map.setFocus(null);` should also be called after a mouse wheel; not only after a touch interaction. Could this be done in ol.map when the tile queue is empty?

See also https://github.com/openlayers/ol3/pull/766#issuecomment-19036947 and #510