Show different icons based on event severity

So far this is only supported in the Suunto EON Steel backend, but we
should try to add this to others where we have such a distinction (and
maybe assign different values to the predefined libdivecomputer events).

This also adds three new icons for info, warning, and violation. The
warning icon we had already, but I drew a new one from scratch to have it
match the violation icon.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

icons/warning-icon.svg

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