Reconsider notification levels (urgency)

[plexus]

Currently the notifier plugin logs failing tests with an "urgency" of "critical", this causes the notifications to become persistent on certain systems (at least KDE, possibly also gnome).

I would like to see two changes
a) a breaking change moving from urgencies "normal" and "critical" to "low" and "normal"
b) making the level configurable

:kaocha.plugin.notifier/urgency :normal

With possible values

• :auto, the default, use low/normal depending on result
• :low / :normal / :critical : use fixed value

Note that this currently only impacts linux.