Gui: Fix invalid scene graph mutation in EditableDatumLabel.

`EditableDatumLabel` uses `activate`/`deactivate` methods for showing or
hiding its widgets in the scene.

However, `activate`/`deactivate` methods can end up being called as part
of a scene graph / Coin action (`SoAction`) handler, in the context of
`SoFCUnifiedSelection`, and this ended up in a Coin warning and eventual
crash due to the scene graph being manipulated (nodes being removed),
which Coin has checks for in debug mode:

```
Coin error in SoGroup::removeChild(): tried to remove non-existent child
0x5555579c5290 (Annotation)
```

Fix this issue by using a `SoSwitch` node instead and by controling the
child visibility using it instead.

src/Gui/EditableDatumLabel.cpp

@@ -27,6 +27,7 @@
 # include <Inventor/nodes/SoAnnotation.h>
 # include <Inventor/nodes/SoOrthographicCamera.h>
 # include <Inventor/nodes/SoTransform.h>
+# include <Inventor/nodes/SoSwitch.h>
 #endif // _PreComp_
 
 #include <Gui/Application.h>
@@ -60,13 +61,17 @@ EditableDatumLabel::EditableDatumLabel(View3DInventorViewer* view,
     , function(Function::Positioning)
 {
     // NOLINTBEGIN
-    root = new SoAnnotation;
+    root = new SoSwitch;
     root->ref();
-    root->renderCaching = SoSeparator::OFF;
+
+    annotation = new SoAnnotation;
+    annotation->ref();
+    annotation->renderCaching = SoSeparator::OFF;
+    root->addChild(annotation);
 
     transform = new SoTransform();
     transform->ref();
-    root->addChild(transform);
+    annotation->addChild(transform);
 
     label = new SoDatumLabel();
     label->ref();
@@ -82,16 +87,19 @@ EditableDatumLabel::EditableDatumLabel(View3DInventorViewer* view,
     if (autoDistance) {
         setLabelRecommendedDistance();
     }
-    root->addChild(label);
+    annotation->addChild(label);
 
     setPlacement(plc);
     // NOLINTEND
+
+    static_cast<SoSeparator*>(viewer->getSceneGraph())->addChild(root); // NOLINT
 }
 
 EditableDatumLabel::~EditableDatumLabel()
 {
     deactivate();
     transform->unref();
+    annotation->unref();
     root->unref();
     label->unref();
 }
@@ -102,7 +110,7 @@ void EditableDatumLabel::activate()
         return;
     }
 
-    static_cast<SoSeparator*>(viewer->getSceneGraph())->addChild(root); // NOLINT
+    root->whichChild = 0;
 
     //track camera movements to update spinbox position.
     auto info = new NodeData{ this };
@@ -129,9 +137,7 @@ void EditableDatumLabel::deactivate()
         cameraSensor = nullptr;
     }
 
-    if (viewer) {
-        static_cast<SoSeparator*>(viewer->getSceneGraph())->removeChild(root); // NOLINT
-    }
+    root->whichChild = SO_SWITCH_NONE;
 }
 
 void EditableDatumLabel::startEdit(double val, QObject* eventFilteringObj, bool visibleToMouse)

src/Gui/EditableDatumLabel.h

@@ -34,6 +34,8 @@
 
 class SoNodeSensor;
 class SoTransform;
+class SoAnnotation;
+class SoSwitch;
 
 namespace Gui {
 
@@ -97,7 +99,8 @@ private:
     SbVec3f getTextCenterPoint() const;
 
 private:
-    SoSeparator* root;
+    SoSwitch* root;
+    SoAnnotation* annotation;
     SoTransform* transform;
     QPointer<View3DInventorViewer> viewer;
     QuantitySpinBox* spinBox;