Transient notifications have used for lots of different "system status"
notifications, like network, low power, low disk space, etc. However, a
majority of these notifications should really also be persistent
instead of going away after they appear.
Users have reported getting confused after seeing a notification appear
up in the corner of their eye, and then have no record of what it was
since the tray was empty.
To simplify the code, set the users more at ease, and also make things
like low power and low disk space more noticeable and urgent after they
go away.
Applications can and should explicitly close any notification it wants
to when state changes, so these notifications shouldn't linger if the
user e.g. plugs in his power cable, or clears up some disk space.
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