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

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/TaskLinkDim.h

@@ -25,19 +25,21 @@
 
 #include <Gui/TaskView/TaskDialog.h>
 #include <Gui/TaskView/TaskView.h>
-#include <Mod/TechDraw/App/DrawViewDimension.h>
-#include <Mod/TechDraw/Gui/ui_TaskLinkDim.h>
 
+class QTreeWidgetItem;
 
 namespace Gui {
 class Document;
 }
 
-class Ui_TaskLinkDim;
+namespace TechDraw {
+class DrawViewDimension;
+}
 
 namespace TechDrawGui
 {
 
+class Ui_TaskLinkDim;
 class TaskLinkDim : public QWidget
 {
     Q_OBJECT