[FIX] base: properly unlink child views

Before this commit:

* Install module A
* Edit a view from module A, add a custom field
* Uninstall module A
* Reinstall module A
* Try to access the module A's previously modified view
    -> Can't access the view, or even sometimes the app at all.

Cause:

When uninstalling module A, the custom field added to module A's is
properly unlinked, however the custom/studio view is not unlinked, therefore
the view references a field that no longer exists, when accessing the
view it will obviously crash (sometimes the traceback is not even
displayed to the user!).

Solution:

Emulate an ondelete cascade for child views only when in uninstall mode.