#838 Create child presets in structures

[arnaudherve]

Possibility for Admin to chose Child checked by defaut, in
Create new page form at the bottom of pages in
structures. That can be useful when Editors have
already arranged structures, and they just want
Registered to create Child pages.

Maybe creation of a new right: Create Child page, that
can be assigned to Registered whereas Editors keep the
right to edit the complete structures.

Well, however it is obvious that with the present form
many users will create pages of the same level instead
of children, and thus mess up with sometimes painfully
elaborated structures.