game_list: Make containsAllWords() a non-member function

This doesn't depend on class state at all, so this can be made internal.
While we're at it, amend the function to take its arguments by const reference.

src/citra_qt/game_list.h

@@ -115,7 +115,6 @@ private:
     void DonePopulating(QStringList watch_list);
 
     void RefreshGameDirectory();
-    bool containsAllWords(QString haystack, QString userinput);
 
     void PopupContextMenu(const QPoint& menu_location);
     void AddGamePopup(QMenu& context_menu, u64 program_id);