Parameterise the suggested extension

[mjakeman]

We currently hardcode an "e.g. Blur my Shell" in the search bar

As much as I do love Blur my Shell, I think it'd be great to rotate between a list of popular extensions (which can be hardcoded for a given release), and maybe even show a different one each time the user opens the browse view.

We should only list extensions which have a fast update cycle for the latest GNOME version, so that users don't find that the suggested extension is unexpectedly missing (since we now filter out unsupported extensions by default).

This will be a breaking change for translations, so let's try get it done for the 0.4 release.