Web widgets: misc fixes

short list of improvements / fixes

• use the nuxeo-core persistence framework in nuxeo-theme-webwidgets /.../ PersistentProvider.java for running transactions and queries

• make hibernate entities serializable

• the <style> parsing of web widgets in IE6+ fails

• add "sticky notes" web widget in samples

• optimize the rendering of widget data such as images (using a server/client cache).
  Serving images currently takes 100-200ms