Update help and appstream information

While at that make sure we build without warnings:

* build: Specify target GTK version

  This avoids warnings until we catched up

* plugins: Remove unused variables

* help: Update notification screenshots

  We don't have a trayicon since ages so use screenshots of the
  notifications
  
  Closes: https://gitlab.gnome.org/GNOME/krb5-auth-dialog/-/issues/14

* build: Switch to appstreamcli

  appstream-util is in deep maintenance mode

* appstream: Modernize

  - Drop detailed content rating list
  - Use https:// URL for screenshot
  - Add launchable

Closes: #14 (closed)