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

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 TD dialogs that don't already use a unique_ptr.

src/Mod/TechDraw/Gui/TaskCosmeticLine.cpp

@@ -126,7 +126,6 @@ TaskCosmeticLine::TaskCosmeticLine(TechDraw::DrawViewPart* partFeat,
 
 TaskCosmeticLine::~TaskCosmeticLine()
 {
-    delete ui;
     if (m_saveCE != nullptr) {
         delete m_saveCE;
     }