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

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/TaskCustomizeFormat.cpp

@@ -39,6 +39,8 @@
 #include <Base/Tools.h>
 #include <Base/UnitsApi.h>
 
+#include <App/DocumentObject.h>
+
 #include <Gui/Application.h>
 #include <Gui/BitmapFactory.h>
 #include <Gui/Command.h>