On empty pgp keyring the key selection dialog would display a label "No Keys available"
which subsequently while selecting different accounts would disappear.
Co-authored-by: Gnoxter <gnoxter+github@linuxlounge.net>
Previously, you had to issue two clicks in order to enter something into
the text fields.
This also replaces the `Entry` widgets with their label counterparts
once they lose focus. Since the `ComboBox` handling of focus seems to be
buggy (`focus_out_event` not firing), the OpenPGP key selection lacks
the latter feature.
