Add the "kDBList::GetSelectedCount" method and "SelectedRecords" tag using it

Plan:

1. in "kDBList::GetSelectedCount" add call to "kDBList::Query" method if list wasn't queried (like it's done in "kDBList::GetRecordsCount") - 0.2h
2. create "kDBTagProcessor::SelectedRecords" tag, that will call above method - 0.2h
3. add "selected" type to "kDBTagProcessor::GridInfo" tag to allow retrieval of selected record count - 0.3h
4. create test plan - 0.3h

Total: 1h