SearchQueryParser support for multiple default fields

## What?
Add support for multiple default fields in SearchQueryParser.

## Why?
SearchQueryParser currently supports only one default field, but people regularly think that the query/search is broken, because they try to search in multiple fields without specifying which one they are using, by simply putting the searched-for word into the query input.

The amount of fields is usually very limited and this parser is used in collections with very low number of items. So even if we'll search in all possible fields at once, the performance should still be ok.

This will greatly simplify the search and the UX of the entity tables.