Gui: Add QtStyle user parameter

This allows theme developers to set proper Qt style.

src/Gui/PreferencePages/DlgSettingsGeneral.cpp

@@ -251,6 +251,9 @@ void DlgSettingsGeneral::saveSettings()
     int blinkTime{hGrp->GetBool("EnableCursorBlinking", true) ? -1 : 0};
     qApp->setCursorFlashTime(blinkTime);
 
+    std::string qtStyle = hGrp->GetASCII("QtStyle");
+    qApp->setStyle(QString::fromStdString(qtStyle));
+
     saveDockWindowVisibility();
 
     hGrp = App::GetApplication().GetParameterGroupByPath("User parameter:BaseApp/Preferences/MainWindow");

src/Gui/StartupProcess.cpp

@@ -229,6 +229,7 @@ void StartupPostProcess::execute()
     setWheelEventFilter();
     setLocale();
     setCursorFlashing();
+    setQtStyle();
     checkOpenGL();
     loadOpenInventor();
     setBranding();
@@ -296,7 +297,6 @@ void StartupPostProcess::setLocale()
     else if (localeFormat == 2) {
         Translator::instance()->setLocale("C");
     }
-
 }
 
 void StartupPostProcess::setCursorFlashing()
@@ -307,6 +307,13 @@ void StartupPostProcess::setCursorFlashing()
     QApplication::setCursorFlashTime(blinkTime);
 }
 
+void StartupPostProcess::setQtStyle()
+{
+    ParameterGrp::handle hGrp = WindowParameter::getDefaultParameter()->GetGroup("General");
+    auto qtStyle = hGrp->GetASCII("QtStyle");
+    QApplication::setStyle(QString::fromStdString(qtStyle));
+}
+
 void StartupPostProcess::checkOpenGL()
 {
     QWindow window;

src/Gui/StartupProcess.h

@@ -65,6 +65,7 @@ private:
     void setWheelEventFilter();
     void setLocale();
     void setCursorFlashing();
+    void setQtStyle();
     void checkOpenGL();
     void loadOpenInventor();
     void setBranding();