Fix 3 issues with the Draft Tray layer list

Fix 3 issues with the Draft Tray layer list:
The layer list should display even if there are no layers so that the "Add new Layer" option can be selected.
"Project" and "BuildingPart" objects should be added to the list (depending on "AutogroupAddGroups" of course).
The "LayerContainer" object should not be added to the list. Although it is in fact a group, it is just too confusing.

Added is_group function. This function can also be used in gui_groups.py.

src/Mod/Draft/draftutils/groups.py

@@ -41,6 +41,34 @@ from draftutils.translate import _tr
 from draftutils.messages import _msg, _err
 
 
+def is_group(obj):
+    """Return True if the given object is considered a group.
+
+    Parameters
+    ----------
+    obj : App::DocumentObject
+        The object to check.
+
+    Returns
+    -------
+    bool
+        Returns `True` if `obj` is considered a group:
+
+        The object is derived from `App::DocumentObjectGroup` but not
+        a `'LayerContainer'`.
+
+        Or the object is of the type `'Project'`, `'Site'`, `'Building'`,
+        `'Floor'` or `'BuildingPart'` from the Arch Workbench.
+
+        Otherwise returns `False`.
+    """
+
+    return ((obj.isDerivedFrom("App::DocumentObjectGroup")
+                and obj.Name != "LayerContainer")
+            or utils.get_type(obj) in ("Project", "Site", "Building",
+                                       "Floor", "BuildingPart"))
+
+
 def get_group_names(doc=None):
     """Return a list of names of existing groups in the document.
 
@@ -54,12 +82,10 @@ def get_group_names(doc=None):
     Returns
     -------
     list of str
-        A list of names of objects that are "groups".
-        These are objects derived from `App::DocumentObjectGroup`
-        or which are of types `'Floor'`, `'Building'`, or `'Site'`
-        from the Arch Workbench.
+        A list of names of objects that are considered groups.
+        See the is_group function.
 
-        Otherwise, return an empty list.
+        Otherwise returns an empty list.
     """
     if not doc:
         found, doc = utils.find_doc(App.activeDocument())
@@ -71,8 +97,7 @@ def get_group_names(doc=None):
     glist = []
 
     for obj in doc.Objects:
-        if (obj.isDerivedFrom("App::DocumentObjectGroup")
-                or utils.get_type(obj) in ("Floor", "Building", "Site")):
+        if is_group(obj):
             glist.append(obj.Name)
 
     return glist