feat: config flag to avoid QtADS using Qt StyleSheet (#835)

Resolve: githubuser0xFFFF/Qt-Advanced-Docking-System#833

demo/MainWindow.cpp

@@ -779,6 +779,9 @@ CMainWindow::CMainWindow(QWidget *parent) :
 	// uncomment if you would like to close tabs with the middle mouse button, web browser style
 	// CDockManager::setConfigFlag(CDockManager::MiddleMouseButtonClosesTab, true);
 
+	// uncomment if you would like to avoid using the built-in QSS stylesheet
+	// CDockManager::setConfigFlag(CDockManager::DisableStylesheet, true);
+
 	// Now create the dock manager and its content
 	d->DockManager = new CDockManager(this);
 	d->DockManager->setDockWidgetToolBarStyle(Qt::ToolButtonIconOnly, ads::CDockWidget::StateFloating);

src/DockManager.cpp

@@ -203,6 +203,10 @@ DockManagerPrivate::DockManagerPrivate(CDockManager* _public) :
 //============================================================================
 void DockManagerPrivate::loadStylesheet()
 {
+	if (CDockManager::testConfigFlag(CDockManager::DisableStylesheet))
+	{
+		return;
+	}
 	initResource();
 	QString Result;
 	QString FileName = ":ads/stylesheets/";

src/DockManager.h

@@ -64,8 +64,13 @@ struct AutoHideTabPrivate;
  * of the docking system. The dock manager uses an internal stylesheet to
  * style its components like splitters, tabs and buttons. If you want to
  * disable this stylesheet because your application uses its own,
- * just call the function for settings the stylesheet with an empty
- * string.
+ * you can either set the DisableStylesheet config flag before creating
+ * the dock manager:
+ * \code
+ * CDockManager::setConfigFlag(CDockManager::DisableStylesheet, true);
+ * \endcode
+ * Or call the function for settings the stylesheet with an empty string
+ * after creating the dock manager:
  * \code
  * DockManager->setStyleSheet("");
  * \endcode
@@ -218,6 +223,7 @@ public:
 		DoubleClickUndocksWidget = 0x10000000, //!< If the flag is set, a double click on a tab undocks the widget
 		TabsAtBottom = 0x20000000, //!< If the flag is set, tabs will be shown at the bottom instead of in the title bar.
 		UseNativeWindows = 0x40000000, //!< If the flag is set, windows for the dock and area widgets will be native.
+		DisableStylesheet = 0x80000000, //!< If the flag is set, the dock manager will not apply the default stylesheet
 
 
         DefaultDockAreaButtons = DockAreaHasCloseButton