[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.ui

@@ -9,8 +9,8 @@
    <rect>
     <x>0</x>
     <y>0</y>
-    <width>360</width>
-    <height>385</height>
+    <width>250</width>
+    <height>352</height>
    </rect>
   </property>
   <property name="sizePolicy">
@@ -371,20 +371,6 @@
      </item>
     </layout>
    </item>
-   <item>
-    <widget class="QCheckBox" name="cbFlip">
-     <property name="enabled">
-      <bool>false</bool>
-     </property>
-     <property name="toolTip">
-      <string>Flips endpoints of selected lines for centerline creation,
-see the FreeCAD Wiki '2LineCenterLine' for a description</string>
-     </property>
-     <property name="text">
-      <string>Flip Ends</string>
-     </property>
-    </widget>
-   </item>
   </layout>
  </widget>
  <customwidgets>