[Part] make pointers to the UI std::unique_ptr

Same as PR #4293, just for Part

as noted in https://github.com/FreeCAD/FreeCAD/pull/4271#discussion_r554673632
the pointer to the UI should be a unique pointer.

This PR does this for all Part dialogs that don't already use a unique_ptr.

src/Mod/Part/Gui/DlgExtrusion.cpp

@@ -139,7 +139,6 @@ DlgExtrusion::~DlgExtrusion()
     }
 
     // no need to delete child widgets, Qt does it all for us
-    delete ui;
 }
 
 void DlgExtrusion::changeEvent(QEvent *e)