Allow switching icon theme in musredit

Added the needed facility in the preferences which allows to switch
the icon scheme between plain and dark. This is needed for some OS
where qt doesn't provide the necessary information.

src/musredit_qt5/musredit/PTextEdit.h

@@ -171,6 +171,7 @@ private:
   QString fLastDirInUse; ///< string holding the path from where the last file was loaded.
   QStringList fMusrFTPrevCmd;
 
+  QMap<QString, QAction*> fActions;
   QAction *fMusrT0Action;
 
   PMsr2DataParam *fMsr2DataParam; ///< structure holding the necessary input information for msr2data
@@ -190,6 +191,8 @@ private:
   void fillRecentFiles();
   QStringList getRunList(QString runListStr, bool &ok);
   bool fileAlreadyOpen(QFileInfo &finfo, int &idx);
+
+  void switchIcons();
 };