search query: add some more clickable options
currently the search is most powerful if you know how to enter the correct queries into the search field.
maybe we want to have some clickable stuff also (see also moin 1.9 FindPage), e.g.
1. what contenttypes to search in (usually it searches in all types) - use groups of stuff so that a normal user understands it (see index page)
2. if farm wikis share a common index, we maybe could display a list of wikis with checkboxes to limit search to the selected wikis
3. maybe offer a list of tags to select