pgp: check encryption keys before sending message

Add check for public keys of all message recipients (to, cc, and bcc)
before sending the message. Adds an OnFocusLost callback to header
editors to facilitate a callback for checking keys whenever a new
recipient is added (OnChange results in too many keyring checks).

Once encryption is initially set, the callbacks are registered. If a
public key is not available for any recipient, encryption is turned off.
However, notably, the callbacks are still registered meaning as s soon
as the user removes the recipients with missing keys, encryption is
turned back on.

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Tested-by: Koni Marti <koni.marti@gmail.com>

commands/compose/encrypt.go

@@ -2,7 +2,6 @@ package compose
 
 import (
 	"errors"
-	"time"
 
 	"git.sr.ht/~rjarry/aerc/widgets"
 )
@@ -29,16 +28,5 @@ func (Encrypt) Execute(aerc *widgets.Aerc, args []string) error {
 	composer, _ := aerc.SelectedTab().(*widgets.Composer)
 
 	composer.SetEncrypt(!composer.Encrypt())
-
-	var statusline string
-
-	if composer.Encrypt() {
-		statusline = "Message will be encrypted."
-	} else {
-		statusline = "Message will not be encrypted."
-	}
-
-	aerc.PushStatus(statusline, 10*time.Second)
-
 	return nil
 }