Expose the map renderer type to the public API

In the case  where the map decides itself which renderer to use. It can
be convenient for the developer to have knowledge of the choice made.
Rename ol.RendererHint to ol.RendererType
Provide ol.RendererType structure through ol.renderer.Map

It was previously in ol.Map, but it could not be provided because
of circular dependencies.
Rename Map.DEFAULT_RENDER_HINTS to Map.DEFAULT_RENDERER_TYPES
Enhance and improve the accessibility example.

Changes to improve the accessibility example:

 - remove the `accesskey` attributes. Use of the `accesskey` is generally not recommended as it tends to cause conflicts with AT software and user settings.
 - Change the scripted "Go to map" accesskey to a regular skiplink.
 - Use script to inject the buttons below the map, if the map were to fail for lack of javascript support the buttons should not be there either. These should be buttons (and not hyperlinks) because they provide an action not a navigation.
 - Update the docs.
 - Add an empty `alt` attribute to the logo since it is strictly decorative (this should probably be done sitewide - which IMO is out of scope for this PR)
Add offset and offsetOrigin in ol.style.Icon
Add parsing of gx:x, gx:y, gx:w and gx:h tags in KML format
Add ol.style.Icon tests for getAnchor and getOrigin
Add ol.format.KML test for complex IconStyle
Merge pull request #2164 from oterral/sprite

Allow the use of sprite image in KML
Merge pull request #2113 from GeoDienstenCentrum/improve-a11y-example

Enhance and improve the accessibility example
Rename ol.Collection#getAt to ol.Collection#item

TouchList, DOMStringList, DOMTokenList, NodeList all have an
`item(index)` function to retrieve an item from a list.
We should do the same.
Merge pull request #2170 from fredj/collection-item

Rename ol.Collection#getAt to ol.Collection#item
Merge pull request #2033 from htulipe/map-renderer-type

Expose map renderer type in the API.