Add some separators to menus and toolbars.
Change menus for style-mode esp. remove some sub-menu levels.

* Move the following items contained in the following sub-menus
  up one level, removing the sub-menu:
  - Edit / Icons
  - Edit / Node extensions
  - Edit / Note
  Reasoning: The style mode is about editing styles. So the resp. actions
  should be available easily.
* Add "Add ....." menu items to map-pop-up.
* Add "Add level style" menu item to node-pop-up.
* Add some separators.
Save menu-mindmaps without modification-time and folding.
Change menus for mindmap-mode esp. new top-level menu "Node".

Reasoning: "Node" is the central concept of the mindmap. There are quite
a lot "extensions" for it. And most time working with the map means
working with nodes. Thus node and it's extensions should be accessable
easily.

* Mode the items contained in the following sub-menus into the new
  top-menu "Node":
  - Edit / Node core
  - Edit / Icons
  - Edit / Node extensions
  - Edit / Note
* Add some separators.
bug fixes in menu builder
Add links to tutorial (wiki) and node-template to menu-mindmaps.