Confusing warning about signing [messages] shown in other contexts

[luke-jr]

The receiving address book tab recently (0.21) added a warning "Signing is only possible with addresses of the type 'legacy'." which is shown even outside the context of signing messages, and even when all addresses are type 'legacy'.

It should only be shown when applicable (some addresses are actually being hidden), and really could benefit from being rephrased.

[luke-jr]

Unfortunately, I don't really see any good way to do this... The hiding logic is terrible as-is, and refactoring it (#157) actually makes it harder to answer the "has anything been hidden" question (because the filtering is done by a const method).