Make passphrase prompt modal to the main window (instead of globally).

addon/content/pEpForThunderbird.js

@@ -71,7 +71,8 @@ class pEpForThunderbird extends pEp {
       ? this.localeMessagesMap.get("passphrasewrong")
       : this.localeMessagesMap.get("passphraserequired");
     let passphraseTitle = this.localeMessagesMap.get("passphrasetitle");
-    let result = promptSvc.promptPassword(null, passphraseTitle, prompt, passphrase, null, check);
+    let win = Services.wm.getMostRecentWindow("mail:3pane");
+    let result = promptSvc.promptPassword(win, passphraseTitle, prompt, passphrase, null, check);
     return result ? passphrase.value : null;
   }