[OMEMO] store contacts' keys (incl. trust) globally, not per account
As it seems, when I have multiple accounts using omemo, which all know some contact $Bob, his key will be stored independently for each of my accounts. This means I have to trust Bobs key once for each of my accounts, which is quite cumbersome (and multiplies if Bob is also using multiple accounts with me).
I therefore suggest storing contact's OMEMO keys independently of local accounts.
This would also remove the UI flaw in the OMEMO config dialog, that the account selection on the first tab filters the lists on the second and third one, without any indication that such filtering exists.