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

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/PreCompiled.h

@@ -69,7 +69,9 @@
 #include <bitset>
 
 #ifdef FC_OS_WIN32
+# define WIN32_LEAN_AND_MEAN
 # include <windows.h>
+# undef small
 #endif
 
 
@@ -77,6 +79,15 @@
 #ifndef __QtAll__
 # include <Gui/QtAll.h>
 #endif
+#include <QPaintEngine>
+#include <QSvgGenerator>
+#include <QGLWidget>
+
+#include <Inventor/nodes/SoDirectionalLight.h>
+#include <Inventor/nodes/SoOrthographicCamera.h>
+#include <Inventor/nodes/SoPerspectiveCamera.h>
+#include <Inventor/nodes/SoSeparator.h>
+#include <Inventor/misc/SoChildList.h>
 
 #endif //_PreComp_