Add a "root parent children" navigation panel

Today, in XWiki, there are a couple of navigation panels that can be configured for a page tree, that provide tree navigation features:

• the standard navigation panel, which displays the whole page tree of the wiki, according to its configuration from the administration (exclusions)
• the children panel, that displays all children of the current page
• the siblings macro, that shows pages on the same level as the current page
• etc.

This issue is about adding a new panel available by default, that would display the navigation inside the first parent from the root of the wiki of the current page.

For example, for a page A.B.C.WebHome, the panel would show a tree with the root in A, without showing A.

For a page P.R.S.WebHome, the panel would show the tree with the root in P, without showing P.

Although a navigation panel that shows a document tree with the root somewhere lower in the tree is both rather easy to write and not restricted to the toplevel pages, this panel would allow to cover a number of usecases, where navigation is expected at first level.

This panel is helpful towards the usecase of setting up "areas" of content in a wiki without creating subwikis, by simulating some form of toplevel units, that are "isolated" between eachother.