[css] drop shadow.gif & use a couple of rules (closes #3381435)

[css] replace old rounded corners hack by css rules Closes #3381426.

[views/templates] provide a modal main template (closes #3274672) The current NonTemplatableViewTemplate may seem to provide the desired feature if a '__notemplate' is provided in the form, unfortunately, it: * wraps the view in a spurious div * calls view.set_stream(), which has the effect of wiping any js/css resource recorded until then by the view, leading to hard-to-track missing resources symptoms