Don't only check parent if isValid, also bindable

The problem was parent always returned true because it was a grid,
but bindable (a form) should also have been validated.
BindableController ask related controller for isValid()

BindableController do not know anything about their content so
they should ask related controller if isValid
Merge pull request #203 from Ben-Ho/3.7-ext4-isValid-bugfix

ext4 is valid bugfix