Change Endless EULA checkbox by button

Ditto.

[endlessm/eos-shell#3594]

I know that it does not seem to fit in with the rest of the internal API, but I really dislike non-constified strings that get copied internally. it makes the whole thing very much error prone, and requires you to fight against the lower levels of the API (like the cast from `g_value_get_string()` that you had to add).