Abstract away InstanceCopyPrefs' internals through new getSelectedFiltersAsRegex() function

+ fix typo in comment
+ remove unused import
+ add [[nodiscard]] to methods

Signed-off-by: Marcelo Hernandez <marcelohdez.inq@gmail.com>

launcher/InstanceCopyTask.h

@@ -24,10 +24,6 @@ protected:
     void copyAborted();
 
 private:
-    // Helper functions to avoid repeating code
-    static void appendToFilter(QString &filter, const QString &append);
-    void resetFromMatcher(const QString &regexp);
-
     /* data */
     InstancePtr m_origInstance;
     QFuture<bool> m_copyFuture;