Explicitly bind xmlns-namespaced attributes.

This fixes XML serialization in IE11. See also #1220 and #1238.
Merge pull request #1239 from marcjansen/xml-ie11-sld

Explicitly bind xmlns-namespaced attributes.
Whitespace cleanup.
Conditionally add features to the selectedfeatures
Remove dangling whitespace.
Promote several properties to API properties.

The properties `multipleKey` and `toggleKey` are used inside the examples
and therefore publicly known anyhow. The properties `onBeforeSelect` and
`scope` are changed to be part of the API for consistency (e.g. `onUnselect`
is part of the API already) and usefullness (we allow the `scope` to be
defined elsewhere in the library as well). The property `renderIntent` is
being promoted as it is very useful and a nice complimentary of the already
public `selectStyle`.
Merge pull request #1241 from marcjansen/select-ctrl-api

Promote several properties to API properties.
Merge pull request #1240 from buehner/conditionallyaddfeatures

Conditionally add features to the selectedfeatures of a layer when using selectFeature of ModifyFeatureControl