Currently, non-trivial client interface customisations generally *have*
to go through client actions (a limited amount can use complex HTML in
the form, but it's not that common). This can require a significant
amount of messing around with JS code, and doesn't integrate well with
the rest of the system (primarily due to the loss of the control panel /
search view).

This change re-uses the existing `ir.ui.view[type=qweb]` as actual views
(bound to a model) to provide for flexible views rendered synchronously
server-side without needing javascript development. A few dynamic
behaviours are also provided (on top of bootstrap's which should be
usable out of the box):

* an extension of `a[type="action"]` which allows for more detailed
  crafting of the action (not just accessing an action by id and by model,
  method)
* a new `a[type="toggle"]` which provides lazy loading of sub-items
  (currently by calling a method which returns HTML to insert after the
  loading anchor)
* simple fold/unfold (without lazy loading) can be performed through
  bootstrap