glade frontend: added new frontend option 'notifications': if True (default), the notifications are enabled.

Also ensure we use cfg.printv() to get the frontend settings, so that we can properly get the boolean values.
semplice-fullscreen now comes with notifications disabled.
live-config: added linstallersession.
Merge branch 'devel' into debian-devel
linstaller-session: add x-session-manager alternative.
5.22.1-1