Change password on IO instead of UI thread

As on Android, we don't use the DB thread, but the IO thread for this.

While the password is changed, a loader is shown, but we don't explicitly prohibit user interaction. When trying to block interaction with a semi-transparent overlay, I run into already known problems coming from the usage of AlertDialog instead of a proper (native) window. Maybe we should prioritize on #236 at some point.

Closing/Dismissing the dialog is prohibited while in the "loading" state, though.

Closes #314 (closed)