Made Arch groups able to receive drops from the tree

src/Mod/Arch/ArchBuilding.py

@@ -68,12 +68,27 @@ class _Building:
     def __init__(self,obj):
         self.Type = "Building"
         obj.Proxy = self
-
+        self.Object = obj
+        
     def execute(self,obj):
-        pass
+        self.Object = obj
         
     def onChanged(self,obj,prop):
         pass
+
+    def addObject(self,child):
+        if hasattr(self,"Object"):
+            g = self.Object.Group
+            if not child in g:
+                g.append(child)
+                self.Object.Group = g
+        
+    def removeObject(self,child):
+        if hasattr(self,"Object"):
+            g = self.Object.Group
+            if child in g:
+                g.remove(child)
+                self.Object.Group = g
         
 class _ViewProviderBuilding:
     "A View Provider for the Building object"