Sketcher: New Feature: External Geometry Linking improvement

============================================================

It lifts the need of clicking on the external geometry icon upon importing each external geometry element.

All the elements on which the user clicks are imported until the user right clicks with the mouse or presses ESC, like the Trim or Fillet tools.

src/Mod/Sketcher/Gui/CommandCreateGeo.cpp

@@ -4270,7 +4270,10 @@ class DrawSketchHandlerExternal: public DrawSketchHandler
 
     virtual bool releaseButton(Base::Vector2D onSketchPos)
     {
-        sketchgui->purgeHandler(); // no code after this line, Handler get deleted in ViewProvider
+        /* this is ok not to call to purgeHandler
+        * in continuous creation mode because the 
+        * handler is destroyed by the quit() method on pressing the
+        * right button of the mouse */
         return true;
     }
 
@@ -4288,7 +4291,10 @@ class DrawSketchHandlerExternal: public DrawSketchHandler
                     Gui::Command::commitCommand();
                     Gui::Command::updateActive();
                     Gui::Selection().clearSelection();
-                    sketchgui->purgeHandler(); // no code after this line, Handler get deleted in ViewProvider
+                /* this is ok not to call to purgeHandler
+                * in continuous creation mode because the 
+                * handler is destroyed by the quit() method on pressing the
+                * right button of the mouse */
                 }
                 catch (const Base::Exception& e) {
                     Base::Console().Error("%s\n", e.what());