Fix parsing of style title from UserStyle.Title element

[giohappy]

Several issues prevent the correct parsing of the style title from the UserStyle.Title element. This is used by MapStore for SLD styles, and is what is returned by Geoserver when converting from CSS (with @styleTitle property) to SLD.

• gs-restconfig dep version must be updated when this PR will be merged and a new version of gs-restconfig will be published
• review the reason of having the geoserver.helpers.style_update method, which is only used inside the geoserver.views.geoserver_proxy method in case of PUT requests. Why is this required if we have the geoserver post save signals?
• if style_update is requested the following must be fixed:
  1. Code will never enter this line, since not elm_user_style_title returns always True even when the element is populated. It must be replaced with elm_user_style_title is None
  2. In case of a CSS style the @styleTitle must be parsed instead