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

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/CommandCreateDims.cpp

@@ -22,6 +22,7 @@
 
 #include "PreCompiled.h"
 #ifndef _PreComp_
+# include <QApplication>
 # include <QMessageBox>
 # include <iostream>
 # include <string>