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/PPrefsDialog.h

@@ -53,8 +53,9 @@ class PPrefsDialog : public QDialog, private Ui::PPrefsDialog
     bool getEnableMusrT0Flag()      { return fEnableMusrT0_checkBox->isChecked(); }
     bool getKeepRunPerBlockChisqFlag() { return fPerRunBlockChisq_checkBox->isChecked(); }
     bool getEstimateN0Flag()        { return fEstimateN0_checkBox->isChecked(); }
-    bool getDarkThemeIconsFlag()    { return fDarkThemeIcons_checkBox->isChecked(); }
-    int  getDump();    
+    bool getDarkThemeIconsMenuFlag()    { return fDarkThemeIconsMenu_checkBox->isChecked(); }
+    bool getDarkThemeIconsToolbarFlag()    { return fDarkThemeIconsToolbar_checkBox->isChecked(); }
+    int  getDump();
     int  getTimeout() { return fTimeout_lineEdit->text().toInt(); }
 
   public slots: