Add an option to only accept messages from contacts

[wanderwildwood]

Closes #1.

@ryran asked for a way to stop a child receiving texts from people who aren't in their contacts. The issue asks for a rebase to pick up SpamBlocker, but a blocklist works backwards for that — it needs you to have already heard from whoever you want to keep out. This is the allow-list version, and it needs no new dependency.

Settings → Blocking → Only allow contacts. Off by default.

The rule sits in QksmsBlockingClient, which already answers the one question the receive workers ask of it, so it covers SMS and MMS through the path they both use. The contacts lookup only runs while the setting is on, so the ordinary case is unchanged.

Three deliberate choices, in case you'd want them differently:

• isBlacklisted() is untouched. A stranger isn't blacklisted, and the conversation UI uses that call to decide whether to show a thread as blocked — answering yes would mark every unknown sender as though it had been blocked by hand.
• It does nothing without the contacts permission. Without it every lookup returns "not a contact", so the rule would block the entire inbox — and it would look like the network had gone quiet rather than a setting misfiring. The decision is a pure function so that case has a test.
• The switch is disabled while a third-party blocking manager is selected, since the rule lives in the built-in client and would otherwise silently do nothing.

Blocked messages still go to the blocked folder rather than being deleted, unless "Drop messages" is also on — for this use case, a message that turns out to matter should be recoverable.

Six tests cover the truth table. Verified on a Mudita Kompakt.

Happy to adjust any of it. I've been keeping a fork current for the Kompakt and would gladly send more upstream if that's useful to you.

[wanderwildwood]

@ryran in case it's useful while this sits: I keep a fork current for the Kompakt and this is in it — https://github.com/wanderwildwood/inkmessage-plus/releases/tag/v1.0.12. Same lineage and licence, own application id, so it installs alongside rather than replacing anything. Not trying to pull anyone away from InkMessage; just so you're not stuck waiting.