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.

f95d0c31d45558c7b3e2fc39398f51eb0190bdca 0.7.0a1
4233be74572a1af8eebd502c009a10641c63ca0c 0.7.0a3
5c28cf8815319abd8c963022091cf4726764a3e6 0.7.0b1
51f73d210da908c9fb292e6bd7fda42a9d2b8453 0.7.0