fix(dbus): refresh AvailableProducts property

[imobachgs]

The HTTP/JSON API uses a proxy to access the products interface. When the language changes, the AvailableProducts property is silently updated (not really updated because it gets calculated each time we try to access it).

The proxy does not know that this property changed and returns the old value. We can:

• Stop caching those values (in the HTTP/JSON API).
• Fix the D-Bus service to report that the AvailableProducts changed (this one).
• Decide that AvailableProperties is a method instead of a property.

This PR is a tentative fix I have not tried in a real ISO.

To do

• After logging in, the wrong translations still appear.
• Fix unit tests

[coveralls]

coverage: 70.914% (-0.004%) from 70.918%
when pulling 862aae2 on fix-products-i18n
into 7b4db54 on master.

[mvidner]

Testing this in a container...
It kind of works, the product descriptions are shown in the new language, but only if I change the language from the main page.
If I change it in the Product page, it stays in the old language even after going back and forth, or changing the product.

[imobachgs]

Thanks for testing. It is still a work in progress, but I will work on that after the new UI is merged.