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

For MSVC this reduces compile time by ~20%

src/Mod/TechDraw/Gui/TaskLineDecor.h

@@ -25,12 +25,8 @@
 
 #include <Gui/TaskView/TaskDialog.h>
 #include <Gui/TaskView/TaskView.h>
-#include <Mod/TechDraw/Gui/ui_TaskLineDecor.h>
-#include <Mod/TechDraw/Gui/ui_TaskRestoreLines.h>   //????
 
 
-class Ui_TaskLineDecor;
-
 namespace App
 {
 class DocumentObject;
@@ -40,6 +36,8 @@ class DocumentObject;
 namespace TechDrawGui
 {
 
+class Ui_TaskLineDecor;
+class Ui_TaskRestoreLines;
 class TaskLineDecor : public QWidget
 {
     Q_OBJECT