TD: [skip ci] activate PCH of TechDraw (Gui) and use forward declarations

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/TaskSectionView.h

@@ -25,13 +25,15 @@
 
 #include <Gui/TaskView/TaskDialog.h>
 #include <Gui/TaskView/TaskView.h>
-#include <Mod/TechDraw/Gui/ui_TaskSectionView.h>
-#include <Mod/TechDraw/App/DrawViewPart.h>
-#include <Mod/TechDraw/App/DrawViewSection.h>
 
 
 class Ui_TaskSectionView;
 
+namespace TechDraw {
+    class DrawViewPart;
+    class DrawViewSection;
+}
+
 namespace TechDrawGui
 {