Mutable symbolizer properties when returned from a style function

This change adds setters for symbolizer properties. In addition, it introduces a mutable flag on all styles. By default, this is set to `true`. `ol.style.createStyleFunction()` sets it to false for all static styles.

The new setters assert that the mutable flag is true, so whenever an application tries to set a symbolizer property on a style that was assigned to a vector layer or feature overlay, the assertion will fail.