Hide the search env switcher by default

[g123k]

Hi everyone!

For the next release of the app, we still want to keep some parts of the OxF integration (with the scanner for example), but we're not ready to promote it yet.

That's why we want to hide the OxF switcher from search:

However, hiding doesn't mean removing it all.
In the preferences, the idea is to add a switch to hide/show the swicher (default value: invisible).

[monsieurtanuki]

That goes beyond hiding those radio buttons: barcode search are performed on all OxF.

[g123k]

That goes beyond hiding those radio buttons: barcode search are performed on all OxF.

We will keep barcode search because @stephanegigandet will provide a unified API next week.
The idea is that we don't want to explicitly show that OxF is in the app, but if you scan something, it will be a surprise.

[monsieurtanuki]

The side-effect may be to consider all products as food, cf. dao_product.dart.
Which isn't the goal, I guess.