Fix non-working sub-search on Front-End

In-Portal search abilities, that allows to search data across all categories and their items (e.g. links, articles, etc.).

Also there is a subsearch feature that allows to further search only within found data, but using different keyword. This usually works, but I've found several problems in subsearch implementation:
1. sub-search on categories doesn't work and gives Fatal Error, that OnSubSearch method is undefined
2. if sub-search is made across multiple item types, that are displayed on same page, then subsearch is performed on first item type being displayed and regular search is performed on other item types (you see more results then were found originally)
3. when during subsearch nothing is found all results (from a previous search) are displayed