Allow getting groups automatically

[Martronic-SA]

That would be nice to find a way to automatically add groups (group of fields) in an form (and autoform) without overriding the form.

my idea:
in fieldsets/extensible.py

class ExtensibleForm(GroupForm):
...
def getGroups(self):
adapters = getAdapters((self.context, self.request, self), IDescriptiveGroup)
groups = [group[1] for group in adapters if group[1].available]
return groups

def update(self):
    self.groups = self.groups + self.getGroups()
    self.updateFields()
    super(ExtensibleForm, self).update()

There could be also a zcml instruction like
<plone:formgroup
name="theform.mycustomgroup"
for="plone.views.IAvailableForm"
layer="IMyBrowserLayer"
class=".mygroups.Mygroup"
order="7"
/>

What do you think?