[TD] fix centerline crashes and bugs

the PR fixes the following things:
- change the way centerlines between 2 lines are calculated. The current implementation leads to many bugs and even crashes (e.g. when the 2 selected lines are parallel ones of a square). There are different forum threads from the last 1.5 years.
The new endpoint line sorting is the one described here: https://forum.freecadweb.org/viewtopic.php?f=35&start=20&t=44255&sid=989a668890f954c13ef81e4a04ee6912#p501179

- as consequence the erroneous and misleading line end flipping can go and is removed (only used internally)

- when creating a new centerline, you see it immediately

- when creating a new or editing an existing centerline and press Cancel, the creation/editing is aborted

- fix crash when the 2 selected lines are bot horizontal and one tries to create a vertical centerline

- fix crash when changing the orientation  of an existing centerline and the result would be invalid

- cleanup the code a bit

src/Mod/TechDraw/Gui/TaskCenterLine.h

@@ -94,24 +94,16 @@ public:
     void saveButtons(QPushButton* btnOK,
                      QPushButton* btnCancel);
     void enableTaskButtons(bool b);
-    void setFlipped(bool b);
 
 protected Q_SLOTS:
 
 protected:
     void changeEvent(QEvent *e);
-
-    void blockButtons(bool b);
+    void setUiConnect(void);
     void setUiPrimary(void);
     void setUiEdit(void);
-
     void createCenterLine(void);
-    void create2Lines(void);
-    void create2Points(void);
-
-    void updateCenterLine(void);
-    void update2Lines(void);
-    void update2Points(void);
+    void updateOrientation(void);
 
     double getCenterWidth();
     QColor getCenterColor();
@@ -127,7 +119,6 @@ private Q_SLOTS:
     void onColorChanged();
     void onWeightChanged();
     void onStyleChanged();
-    void onFlipChanged();
 
 private:
     std::unique_ptr<Ui_TaskCenterLine> ui;
@@ -144,6 +135,7 @@ private:
     double m_extendBy;
     int m_geomIndex;
     TechDraw::CenterLine* m_cl;
+    TechDraw::CenterLine orig_cl;
     int m_clIdx;
     int m_type;
     int m_mode;