Introduce "menu" concept

Imported From: http://groups.google.com/group/in-portal-dev/browse_thread/thread/73bfa7f84417db39#

Right now all sections (former categories) have "Show In Menu" checkbox and you can set their menu order from dropdown.

This seems good unless you add up several hundreds product categories to a catalog, which instantly would have their Priority field populated and default priority sorting that in pre In-Portal 5.x versions allowed to show
some categories on top now would only force category order and regular sorting by category name won't work anymore.

I'm proposing to:

Add "Menus" section where you can create as many menus as you like.
Each menu can consist of unlimited number of items.
Each menu item can have:
- reference to a section (category) in catalog (clicking on such menu would redirect user to a page, that corresponds to that category)
- reference to other menu, that would act as submenu to that menu (this way you can have one menu be a sub-menu of multiple other menus; sort of related categories)
Make Priority/Order field in Categories table an input box again (not a dropdown as before).
Reset priority of all categories (of that's I'm not 100% sure)
Automatically create corresponding menus for categories that has "Show In Menu" checkbox checked and remove this checkbox from Categories table

This approach would allow to have menus of any level of complexity, that would at the end point out to section/categories in "Structure & Data" section, that represent pages of website. Also adding thousands of categories won't slow down a menus at all.

In past we have problems due menus being merged with categories in In-Portal 5.x resulting to much misunderstandings from both developer and website owner sides when time came to use them.