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/PPrefsDialog.h

@@ -53,6 +53,7 @@ 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();    
     int  getTimeout() { return fTimeout_lineEdit->text().toInt(); }