Jump to correct tab when introducing new lemmas.

[TentativeConvert]

Example: In NNG > Tutorial World > Level 4, a lemma one_eq_succ_zero becomes newly available. However, this is not visible, as it appears in a hidden tab:

It would be helpful if the relevant tab was opened automatically:

[joneugster]

Done. If multiple lemmas are added to different tabs in one level, it is undefined which of these tabs is chosen by default. (i.e. just the first one it finds)