Use field sortTitle when sorting by title

The title field uses field type text which does not work for sorting.
The sortTitle field has been introduced to work around that by
duplicating the title field's content into a string field.
Add class phpDoc for query modifiers
[BUGFIX] Prevent Magento from persisting the sort order on new searches

This caused the order select box to show the previously selected sort
field when starting a new search.