Mutable symbolizer properties for style functions

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.