Draft: migrate Layer object and function to the new structure

Move `make_layer` to `draftmake`; `Layer` and `LayerContainer`
to `draftobjects`; `ViewProviderLayer` and `ViewProviderLayerContainer`
to `draftviewproviders`.

The make function and the classes are imported in `Draft.py`
to support the usage of the older `VisGroup`.

src/Mod/Draft/draftobjects/layer.py

@@ -0,0 +1,120 @@
+# ***************************************************************************
+# *   Copyright (c) 2014 Yorik van Havre <yorik@uncreated.net>              *
+# *   Copyright (c) 2020 Eliud Cabrera Castillo <e.cabrera-castillo@tum.de> *
+# *                                                                         *
+# *   This file is part of the FreeCAD CAx development system.              *
+# *                                                                         *
+# *   This program is free software; you can redistribute it and/or modify  *
+# *   it under the terms of the GNU Lesser General Public License (LGPL)    *
+# *   as published by the Free Software Foundation; either version 2 of     *
+# *   the License, or (at your option) any later version.                   *
+# *   for detail see the LICENCE text file.                                 *
+# *                                                                         *
+# *   This program is distributed in the hope that it will be useful,       *
+# *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
+# *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
+# *   GNU Library General Public License for more details.                  *
+# *                                                                         *
+# *   You should have received a copy of the GNU Library General Public     *
+# *   License along with this program; if not, write to the Free Software   *
+# *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  *
+# *   USA                                                                   *
+# *                                                                         *
+# ***************************************************************************
+"""Provides the object code for the Layer object."""
+## @package layer
+# \ingroup draftobjects
+# \brief Provides the object code for the Layer object.
+
+## \addtogroup draftobjects
+# @{
+from PySide.QtCore import QT_TRANSLATE_NOOP
+
+
+class Layer:
+    """The Layer object.
+
+    This class is normally used to extend a base `App::FeaturePython` object.
+    """
+
+    def __init__(self, obj):
+        self.Type = "Layer"
+        self.Object = obj
+        self.set_properties(obj)
+
+        obj.Proxy = self
+
+    def onDocumentRestored(self, obj):
+        """Execute code when the document is restored.
+
+        Add properties that don't exist.
+        """
+        self.set_properties(obj)
+
+    def set_properties(self, obj):
+        """Set properties only if they don't exist."""
+        if "Group" not in obj.PropertiesList:
+            _tip = QT_TRANSLATE_NOOP("App::Property",
+                                     "The objects that are part of this layer")
+            obj.addProperty("App::PropertyLinkList",
+                            "Group",
+                            "Layer",
+                            _tip)
+
+    def __getstate__(self):
+        """Return a tuple of objects to save or None."""
+        return self.Type
+
+    def __setstate__(self, state):
+        """Set the internal properties from the restored state."""
+        if state:
+            self.Type = state
+
+    def execute(self, obj):
+        """Execute when the object is created or recomputed. Do nothing."""
+        pass
+
+    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
+
+
+# Alias for compatibility with v0.18 and earlier
+_VisGroup = Layer
+
+
+class LayerContainer:
+    """The container object for layers.
+
+    This class is normally used to extend
+    a base `App::DocumentObjectGroupPython` object.
+    """
+
+    def __init__(self, obj):
+        self.Type = "LayerContainer"
+        obj.Proxy = self
+
+    def execute(self, obj):
+        """Execute when the object is created or recomputed.
+
+        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
+
+    def __getstate__(self):
+        """Return a tuple of objects to save or None."""
+        if hasattr(self, "Type"):
+            return self.Type
+
+    def __setstate__(self, state):
+        """Set the internal properties from the restored state."""
+        if state:
+            self.Type = state
+
+## @}