Fix exception in bpy_extras.object_utils.object_data_add(..)

Adding object-data that doesn't support edit-mode would raise an
exception when the "Enter Edit Mode" preferences was enabled.

Other changes:

- Don't attempt to enter edit-mode for library-data.
- Support entering edit-mode for grease-pencil objects.

Alternate fix for the issue raised by D15999.

release/scripts/modules/bpy_extras/object_utils.py

@@ -142,8 +142,17 @@ def object_data_add(context, obdata, operator=None, name=None):
         bpy.ops.object.mode_set(mode='EDIT')
     else:
         layer.objects.active = obj_new
-        if obdata and context.preferences.edit.use_enter_edit_mode:
-            bpy.ops.object.mode_set(mode='EDIT')
+        if context.preferences.edit.use_enter_edit_mode:
+            if obdata and obdata.library is None:
+                obtype = obj_new.type
+                mode = None
+                if obtype in {'ARMATURE', 'CURVE', 'CURVES', 'FONT', 'LATTICE', 'MESH', 'META', 'SURFACE'}:
+                    mode = 'EDIT'
+                elif obtype == 'GPENCIL':
+                    mode = 'EDIT_GPENCIL'
+
+                if mode is not None:
+                    bpy.ops.object.mode_set(mode=mode)
 
     return obj_new