cubicweb #2443038 synchronous tabs [validation pending]

currently tabs are lazy by design. In some cases it may be desired or necessary to have synchronous tabs, for instance when tabs hold forms posting to themselves. A simple 'lazy' class boolean flag should control that behaviour

done in3.15.3
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closed by#5161573ba806 [tabbed view] make tabs optionaly lazy: lazy=False on the tabbed view trigger synchronous tabs. Closes #2443038