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

Same as PR #4293, just for Sketcher

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 Sketcher dialogs that don't already use a unique_ptr.

src/Mod/Sketcher/Gui/TaskSketcherElements.cpp

@@ -342,7 +342,6 @@ TaskSketcherElements::~TaskSketcherElements()
     }
 
     connectionElementsChanged.disconnect();
-    delete ui;
 }
 
 void TaskSketcherElements::onSelectionChanged(const Gui::SelectionChanges& msg)