Cycle references left on ModelView after the UI is disposed

In this example:
```
from traits.api import *
from traitsui.api import *
import weakref

class SomeModel(HasTraits):
number = Int()

class SomeView(ModelView):
other = Button()
view = View(Item('other'))

def _create_ephemeral():
model = SomeModel()
view = SomeView(model=model)
view.configure_traits()
return model, view

if __name__ == '__main__':
model, view = _create_ephemeral()
view_ref = weakref.ref(view)
model_ref = weakref.ref(model)

del model
del view

print(view_ref())
print(model_ref())

import gc
gc.collect()
print("After GC")
print(view_ref())
print(model_ref())
```

The output is the following:
```
<__main__.SomeView object at 0x7fbf65b24b48>
<__main__.SomeModel object at 0x7fbf65b24830>
After GC
None
None
```

Expected output is this:
```
None
None
After GC
None
None
```

This indicates cycle references being created for the ModelView (and possibly the model). Ideally these references should be cleaned up.

(Preliminary finding: Setting `view.info = None` after `configure_traits` resolves part of the issue, but not all of it.)