two-step: Add a per mode setting to suppress messages

two-step: Add a per mode setting to suppress messages

The messages passed to plymouth display-message can be quite verbose, esp.
in the offline-updates case. Combined with some themes now showing their
own prominent title message explaining what is going on this leads to
undesirable repetitive text being shown.

This commit adds support for a per mode SuppressMessages setting which
allows themes to suppress messages passed to plymouth display-message
on a per mode basis.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>