make the theme handling in musredit more flexible. At the same time adopted some part to more modern c++.

src/musredit_qt6/musredit/PPrefsDialog.h

@@ -55,6 +55,7 @@ class PPrefsDialog : public QDialog, private Ui::PPrefsDialog
     bool getKeepRunPerBlockChisqFlag() { return fPerRunBlockChisq_checkBox->isChecked(); }
     bool getEstimateN0Flag()        { return fEstimateN0_checkBox->isChecked(); }
     bool getYamlOutFlag()           { return fYamlOut_checkBox->isChecked(); }
+    bool getIgnoreThemeAutoDetection() { return fIgnoreThemeAutoDetection_checkBox->isChecked(); }
     bool getDarkThemeIconsMenuFlag()    { return fDarkThemeIconsMenu_checkBox->isChecked(); }
     bool getDarkThemeIconsToolbarFlag()    { return fDarkThemeIconsToolbar_checkBox->isChecked(); }
     bool getOneToOneFlag() { return fOneToOne_checkBox->isChecked(); }