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.h

@@ -23,13 +23,11 @@
 #ifndef TECHDRAWGUI_TASKACTIVEVIEW_H
 #define TECHDRAWGUI_TASKACTIVEVIEW_H
 
-#include <QPushButton>
-
 #include <Gui/TaskView/TaskDialog.h>
 #include <Gui/TaskView/TaskView.h>
-#include <Mod/TechDraw/Gui/ui_TaskActiveView.h>
 
 
+class QPushButton;
 class Ui_TaskActiveView;
 
 namespace TechDraw