Introducing menu types

[nWidart]

This is the PR #24 integrated into 2.0 as well as some minor fixes to it.

[Nitpick-CI]

• Each class must be in a namespace of at least one level (a top-level vendor name)
• Opening brace of a class must be on the line after the definition

[nWidart]

Pending issues:

• if an item had a parent, which generated a URI like page1/menu_item, setting it as root, doesn't update URI
• Moving an item that was a root item, back as a child to another page, removes the correct uri
  • makes it menu_item
  • should be page1/menu_item
• Changing the page of a menu item: fails
• Creating a new menu item, and selecting a parent_id does nothing
• Drag & dropping menu items from the menu edit view, doesn't generate the full URI (page1/menu_item)

Big issue: the menu item is fetched, before even saving, which always results in bad state. A second save is often needed to actually perform the correct operation, since it fetches old data from the database.

[nWidart]

• Update uris on page slug change (all translations)
• Double check all possible combinations of parent/child
• Update drag&drop to use new uri things
• Don't load items in parent dropdown that are already parent of current menuItem