Improve selected extensions "cart" in the generate button

[ia3andy]

Currently, the rocket icon which was meant to represent extensions (or a starter application) is used only here. Also, having 0 shown is not entirely true as 0 means the default (Rest).

My suggestion is to make it clearer:

When it's zero it would show something like:

[ia3andy]

cc @tqvarnst @cescoffier

[ia3andy]

Another option would be to have an icon to represent extensions in all Quarkus. With time, that would give user an intuitive clue when we want to refer to extensions without taking much space.

[ia3andy]

We could also create our own "extension cart" icon, that would be nice :)
I've tried a simple cart and that feels a bit weird.