Fix menu creation if two dividers are in a menu

For example a menu definition like this:
```
"display_tools/---Viewport",
"display_tools/center_twodee",
"display_tools/center_twodee_remove",
"display_tools/center_twodee_ui",
"display_tools/---",
"display_tools/cycle_active_viewport_display_preset_forward",
"display_tools/cycle_active_viewport_display_preset_backward",
"display_tools/---",
"display_tools/toggle_viewport_geom",
"display_tools/toggle_viewport_ctrls",
"display_tools/toggle_viewport_imgplns",
```
Before this bug fix, one of the '"display_tools/---",' dividers would
not be created.

Author: david-cattermole

python/mmSolver/tools/mmmenu/lib.py

@@ -153,7 +153,10 @@ def create_items(items_to_create, main_parent):
     created_items = {}
     for key, parent_key, func_def, is_sub_menu in items_to_create:
         menu_item = created_items.get(key)
-        if menu_item is not None:
+        # We are able to have the same key multiple times if it is a
+        # divider without a name.
+        divider = key.endswith('---')
+        if menu_item is not None and divider is not True:
             continue
         parent = created_items.get(parent_key)
         if parent is None:

python/mmSolver/tools/mmshelf/lib.py

@@ -206,7 +206,10 @@ def create_items(items_to_create, main_parent):
     created_items = {}
     for key, parent_key, func_def, is_first_item, is_last_item in items_to_create:
         items = created_items.get(key)
-        if items is not None:
+        # We are able to have the same key multiple times if it is a
+        # divider without a name.
+        divider = key.endswith('---')
+        if items is not None and divider is not True:
             continue
 
         parents = created_items.get(parent_key)