cubicweb #1579188 reimplement ui message handling [open]

There currently exists a message zone in the top part of the screen. This is used to convey short information snippets such as "Welcomme, Foo" or more often, edition/creation success/failure messages.

However this is implemented in a very ad-hoc fashion (code scattered around web/request, web/controller, forms), it is basically impossible to customize it or inhibit some parts of it. Also it does not work well or at all in the context of mobile apps (using e.g. it gets dropped for whatever reason, and ui-wise it is inappropriate).

We should have something sound in place allowing easy extension/customization (an API ...).

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