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
Rename ol.Observable#dispatchChangeEvent() to #changed()

Since dispatchChangeEvent does not only dispatch a change event, but
also increases the revision counter, the method name should reflect
this.
Update docs for changes proposed in #2684
Upgrade closure-library and closure-util version
Add missing goog.require
Remove mousewheel event name workaround

Fixed upstream https://github.com/google/closure-library/pull/308
Set goog.array.ASSUME_NATIVE_FUNCTIONS to true
Auto configure ol.format.GML if not configured with a featureNS/featureType
Merge pull request #2568 from bartvde/gml-autoconfigure

re-implement auto-configure code for ol.format.GML (r=@ahocevar)
Mention the need to re-render in a single place only
Merge pull request #2678 from ahocevar/mutable-styles

Mutable symbolizer properties for styles
Merge pull request #2684 from ahocevar/notify-change

Rename ol.Observable#dispatchChangeEvent() to #changed()
Remove Google Maps example
Merge pull request #2711 from elemoine/remove-gmaps-example

Remove Google Maps example
Merge pull request #2549 from fredj/assume-array-native

Use goog.array.ASSUME_NATIVE_FUNCTIONS define
Merge pull request #2647 from fredj/closure-lib-upgrade

Upgrade closure library version
Set the opacity directly to element.style.opacity

goog.style.setOpacity handles the IE8 case (but we are doing the same
above in the same function) and Firefox prior to version 3.5 (latest
stable release in April 2011).

See https://developer.mozilla.org/en-US/docs/Web/CSS/opacity