Rely on searchcontext scaffolding instead of creating search query by hand

[micschk]

The current construction of the searchquery assumes a 1:1 field mapping and fails in case of 'match_any' search (across fields). Instead of trying to construct a full searchquery by hand it would be better to simply rely on searchcontext to resolve this.

OP\SearchController:

    [...]
    public function buildsearchquery(HTTPRequest $request)
    {
        $SearchedDataObject = Injector::inst()->create($request->getVar('ClassName'));

        // check for a first-name last-name pattern with people's names
        [...] (think this members-search could/will also be covered by scaffolded searchcontext)

        // Search using searchcontext scaffolding...
        /** @var SearchContext $searchContext */
        if ($searchContext = $SearchedDataObject->getDefaultSearchContext()) {
            $searchParams = [ $SearchedDataObject->getGeneralSearchFieldName() => $request->getVar('query') ];
            return $searchContext->getResults($searchParams);
        }
        
        [...] (DONE - remaining code can probably be removed...)

    }