Close USB device added notification when devicenotifier pops up

Part of plasma-desktop#149.

Reason for the change

The device notification is supposed to be super quick feedback that "something" got detected. Once it has been identified as storage device (disks spun up and what not), devicenotifier will show up and then you have two popups.

Test plan

Enabled popup for device added notification, plugged in a USB stick.

Got a notification about the USB stick, three or so seconds later, it closed and devicenotifier opened. There’s a tiny overlap of the popup starting to open while the notification is closing but not sure if we can really address this other than doing an awful callLater for opening the popup or something.