[DeviceNotifications] Keep last message for each device
Ejecting e.g. a USB stick removes it from the bus (bus power is removed),
but the last notification should be kept, otherwise a message like
"The device can be savely removed" immediately disappears.
Unfortunately it is almost impossible to tell when a device has been
physically removed (only indirectly, when a new device is plugged into
the same port), so allow some small leakage - in case the same device is
later replugged, no leakage happens.
1.insert medium, mount, unmount
-> message "Device can be savely removed" is shown in device notifier
for 5 seconds
- eject medium, reinsert -> device appears without message and with default label
Reviewers: Plasma, broulik, ngraham
Reviewed By: ngraham
Differential Revision: https://phabricator.kde.org/D22312