This commit introduces a new module system for the javascript code.
Instead of using global (namespaced) variables everywhere (like
openerp.web.whatever, ...), each file declare one (or more) module
with its dependencies.  Each module will be loaded after its
dependencies, and will be given a reference to the return values.

This change is a massive departure from the way the client worked
previously.  It will sadly break some/most history, but I am sure that
the future git ecosystem will allow you to easily browse the history
past this revision (one can dream).

This commit feature:
* main namespace is now odoo instead of openerp
* a way to define modules and dependencies (inspired by requirejs)
* many files have been splitted in simpler subfiles
* basic support for debugging in odoo.__DEBUG__
* no global variable (i.e. nothing in window.openerp)
* many bug fixes, many new bugs

Future vision: this modularization should simplify future code
development, simplify debugging and will allow a serious testing effort

The menu was not present in 8.0 when going back from the mass mailing email
composer to the backend. This fix adds the current action id as a return_action
query string parameter and return to the correct action.

fixes #5591

opw-629526

27a48f8026f824f62955d5bf2a83b2799ad98db1 introduced the use of self.dataset
It appears it is not always defined.
Few lines above, self.getParent().dataset is used,
and looks to be always defined.
We therefore now use this dataset in order to get the context,
to correctly pass the current session language.

Closes #5416