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

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/TaskActiveView.cpp

@@ -23,13 +23,13 @@
 #include "PreCompiled.h"
 
 #ifndef _PreComp_
-
-#endif // #ifndef _PreComp_
-
 #include <QApplication>
+#include <QPushButton>
 #include <QStatusBar>
 #include <QGraphicsScene>
 #include <QTemporaryFile>
+#endif // #ifndef _PreComp_
+
 
 #include <Base/Console.h>
 #include <Base/Tools.h>