cagou widget: don't add header_input systematically anymore: header input is often used in CagouWidget for e.g. filtering, but not always, and when it is not, it is weird and confusing to have a TextInput which doesn't do nothing. To avoid that, a new `use_header_input` bool attribute (set to False by default) has been added to CagouWidget.

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .