added an option to enable webgl

webgl can be enabled simply via the ```enable-accelerated-compositing``` option of WebKitWebSettings.

This commit features only an option which, if present, makes webgl enables on gtkwebkit. I could write a more generic implementaion allowing to customize webkitgtk more freely but as I don't have time to test them, it might not be better to add all options blindly.

Feel free to suggest a better way to enable this option.