Predefined extents

This patch addes a listbox in the search page where bookmarks (predefined extents) are loaded. The user can choose on of the bookmarks to quickly pan on the map. The extents are loaded from an xml file.
There are some possibile enhancements that can be done:
- the name of the file is hardcoded (gpt/config/extents.xml)
- the WKID is hardcoded (4326)
Example of this funciont can be seen in:
http://www.geoportale.isprambiente.it/geoportale/catalog/main/home.page