More flexible dark scheme handling

It works now as follows:
1) try to get theme information from the system, and if present us it.
2) if no theme information of the system is present, take the one from
   the musredit_startup.xml.

Now it is possible to switch the icon theme for the menu and toolbar
separately.

src/musredit_qt5/musredit/PTextEdit.h

@@ -162,8 +162,8 @@ private slots:
   void setFileSystemWatcherActive();
 
 private:
-  bool fDarkTheme;
-  bool fDarkToolBarIcon;
+  bool fDarkMenuIcon; ///< flag indicating if a dark or plain icon shall be used in the menu pull-downs
+  bool fDarkToolBarIcon; ///< flag indicating if a dark or plain icon shall be used in the toolbar
   PAdmin *fAdmin; ///< pointer to the xml-startup file informations. Needed for different purposes like default working- and executable directories etc.
   QFileSystemWatcher *fFileSystemWatcher; ///< checks if msr-files are changing on the disk while being open in musredit.
   bool fFileSystemWatcherActive; ///< flag to enable/disable the file system watcher
@@ -192,7 +192,8 @@ private:
   QStringList getRunList(QString runListStr, bool &ok);
   bool fileAlreadyOpen(QFileInfo &finfo, int &idx);
 
-  void switchIcons();
+  void switchMenuIcons();
+  void switchToolbarIcons();
 };