Make move callout tool play nice with CAD dock

Means that if the cad dock is enabled, callout lines will auto-snap
to nice angles, etc.

src/app/labeling/qgsmaptoolmovelabel.cpp

@@ -131,6 +131,11 @@ void QgsMapToolMoveLabel::cadCanvasPressEvent( QgsMapMouseEvent *e )
       mCalloutMoveRubberBand->setWidth( 3 );
       mCalloutMoveRubberBand->show();
 
+      // set initial cad point as the other side of the callout -- NOTE we have to add two points here!
+      cadDockWidget()->addPoint( mCurrentCalloutMoveOrigin ? mCurrentCallout.destination() : mCurrentCallout.origin() );
+      cadDockWidget()->addPoint( e->mapPoint() );
+      cadDockWidget()->releaseLocks( false );
+
       return;
     }
     else
@@ -348,6 +353,14 @@ void QgsMapToolMoveLabel::cadCanvasReleaseEvent( QgsMapMouseEvent * )
   }
 }
 
+void QgsMapToolMoveLabel::canvasReleaseEvent( QgsMapMouseEvent *e )
+{
+  if ( mCalloutMoveRubberBand )
+    return; // don't allow cad dock widget points to be cleared after starting to move a callout endpoint
+
+  QgsMapToolLabel::canvasReleaseEvent( e );
+}
+
 void QgsMapToolMoveLabel::keyPressEvent( QKeyEvent *e )
 {
   if ( mLabelRubberBand || mCalloutMoveRubberBand )

src/app/labeling/qgsmaptoolmovelabel.h

@@ -35,6 +35,7 @@ class APP_EXPORT QgsMapToolMoveLabel: public QgsMapToolLabel
     void cadCanvasMoveEvent( QgsMapMouseEvent *e ) override;
     void cadCanvasPressEvent( QgsMapMouseEvent *e ) override;
     void cadCanvasReleaseEvent( QgsMapMouseEvent *e ) override;
+    void canvasReleaseEvent( QgsMapMouseEvent *e ) override;
     void keyPressEvent( QKeyEvent *e ) override;
     void keyReleaseEvent( QKeyEvent *e ) override;