Draft: allow objects in layers to have overrides (#19207)

* Draft: allow objects in layers to have overrides

Fixes #17844.

1. When an object is put in a layer it always adopts the properties of that layer.
2. Properties of the object that are subsequently changed are considered overrides.
3. When the properties of the layer, or the object in the layer, are then changed so that they match again, there is no longer an override, and properties are synced again.
4. The layer and the layer container object get an extra Tree view context menu option "Reassign properties of layer"/"Reassign properties of all layers".

src/Mod/Draft/draftobjects/layer.py

@@ -113,12 +113,49 @@ class Layer:
         """Execute when the object is created or recomputed. Do nothing."""
         pass
 
+    def _get_other_layers(self, obj, child):
+        other_lyrs = []
+        for find in child.Document.findObjects(Type="App::FeaturePython"):
+            if utils.get_type(find) == "Layer" and find != obj and child in find.Group:
+                other_lyrs.append(find)
+        return other_lyrs
+
+    def onBeforeChange(self, obj, prop):
+        if prop == "Group":
+            self.oldGroup = obj.Group
+
+    def onChanged(self, obj, prop):
+        if prop != "Group":
+            return
+        vobj = getattr(obj, "ViewObject", None)
+        old_grp = getattr(self, "oldGroup", [])
+        for child in obj.Group:
+            if child in old_grp:
+                continue
+            for other_lyr in self._get_other_layers(obj, child):
+                other_grp = other_lyr.Group
+                other_grp.remove(child)
+                other_lyr.Group = other_grp
+            if vobj is None:
+                continue
+            for prop in ("LineColor", "ShapeAppearance", "LineWidth", "DrawStyle", "Visibility"):
+                vobj.Proxy.change_view_properties(vobj, prop, old_prop=None, targets=[child])
+
     def addObject(self, obj, child):
-        """Add an object to this object if not in the Group property."""
-        group = obj.Group
-        if child not in group:
-            group.append(child)
-        obj.Group = group
+        if utils.get_type(child) in ("Layer", "LayerContainer"):
+            return
+        grp = obj.Group
+        if child in grp:
+            return
+        grp.append(child)
+        obj.Group = grp
+
+    def removeObject(self, obj, child):
+        grp = obj.Group
+        if not child in grp:
+            return
+        grp.remove(child)
+        obj.Group = grp
 
 
 # Alias for compatibility with v0.18 and earlier
@@ -147,9 +184,9 @@ class LayerContainer:
         Update the value of `Group` by sorting the contained layers
         by `Label`.
         """
-        group = obj.Group
-        group.sort(key=lambda layer: layer.Label)
-        obj.Group = group
+        grp = obj.Group
+        grp.sort(key=lambda layer: layer.Label)
+        obj.Group = grp
 
     def dumps(self):
         """Return a tuple of objects to save or None."""
@@ -162,7 +199,6 @@ class LayerContainer:
             self.Type = state
 
 
-# Similar function as in view_layer.py
 def get_layer(obj):
     """Get the layer the object belongs to."""
     finds = obj.Document.findObjects(Name="LayerContainer")