Central Widget concept added

Adde option to set a dock widget as central widget. It influences resizing behavior of the splitters. The central widget will be stretched with the main window and remaing dock widgets and threir respective areas will be resized only vertically if docked left or right  and horizontaly if docked top or bottom

src/DockWidget.cpp

@@ -461,6 +461,12 @@ void CDockWidget::setMinimumSizeHintMode(eMinimumSizeHintMode Mode)
 }
 
 
+bool CDockWidget::isCentralWidget()
+{
+    return dockManager()->centralWidget() == this;
+}
+
+
 //============================================================================
 void CDockWidget::toggleView(bool Open)
 {