Empty notebook pages should be hidden

Some modules add empty notebook pages. They should be automatically hidden either
by the client or the server.

added 1 deleted label

The attached review implements it

assigned to @pokoli

added type::feature + 1 deleted label and removed 1 deleted label

As commented on the old review8721002 :

It is better to deal with that on client side because it will be better
to also hide if it is empty because all fields inside are invisible.

unassigned @pokoli

added sao tryton + 1 deleted label and removed 1 deleted label

New review31061002 at https://codereview.tryton.org/31061002/#ps1

assigned to @xcodinas

added 1 deleted label and removed 1 deleted label

I added review31061002 for sao and review27971002 for tryton that hide the page when it's empty or all fields are hidden.

New review27971002 at https://codereview.tryton.org/27971002/#ps1

I do not think it deserve to add code on the client side.
The case of a page empty because all widgets inside are invisible could be managed by the developer by setting a correct states for the page.
So I see only the case of a module adding an empty page because other dependencies will fill it. And it is done on the root module to avoid conflict. This case can be managed on the server side by removing empty page from the XML of the view.

New review40421002 at https://codereview.tryton.org/40421002/#ps20001

review40421002 updated at https://codereview.tryton.org/40421002/#ps10008

review40421002 updated at https://codereview.tryton.org/40421002/#ps50001

review40421002 updated at https://codereview.tryton.org/40421002/#ps80001

review40421002 updated at https://codereview.tryton.org/40421002/#ps100001

review40421002 updated at https://codereview.tryton.org/40421002/#ps120001

New changeset 0edc59acc20e by Nicolas ?vrard in branch 'default':
Remove empty pages from notebook
http://hg.tryton.org/trytond/rev/0edc59acc20e