Key exchanges in SDES can only be done securely with TLS and the option
is disabled by default if not using TLS as the transport protocol.
This setting allows to override this behaviour if the user desires
it (f.e. if the user considers the network his packets go through to be trusted).
This breaks the settings binding cycle for the "autoload-plugins" and
"preferred-audio-codecs" settings which went something like this:
g_settings_bind () is used with
G_SETTINGS_BIND_DEFAULT (G_SETTINGS_BIND_GET | G_SETTINGS_BIND_SET).
It grabs the value of our setting and sets it for the bound property.
This emits the notify signal causing the same value to be set for the
setting.
In effect this caused the setting to be marked as non-default because
Calls had changed the setting without user action. This caused updated
defaults not to apply for existing installations.
We only have a single source of settings, so we should reflect that by
using a singleton. This also reduces our LoC.
This doesn't impair our ability to run tests because there we run with
GSETTINGS_BACKEND=memory
g_settings_get_string() and g_settings_get_strv() are transfer full,
so we should use g_value_take_string() instead of g_value_set_string() and
g_value_take_boxed() instead of g_value_set_boxed()
