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.
Additional documentation
No need to have a mutable_ flag

Instead, educate users to call setStyle.
Doc improvements
Update docs for changes proposed in #2684
Mention the need to re-render in a single place only
Merge pull request #2678 from ahocevar/mutable-styles

Mutable symbolizer properties for styles