Refactor Gajim startup

• Move everything important for Startup into do_startup where it belongs
• do_handle_local_options handles command options before do_startup
• do_command_line handles command options after do_startup or options passed to Gajim when Gajim is already running. This means we can now change logging options on the fly without restarting Gajim
• Move the whole startup related code back to gajim.py and get rid of application.py