[Device Notifier] Auto-hide popup after device is unmounted

Summary:
When unmounting a removable device from dolphin, the device notifier
will popup saying the device can now be safely removed. But the popup
never auto-hides.

Test Plan:
Plug a USB pen drive
Unmount it from dolpin
Device notifier should popup and hide itself after 3 seconds

Reviewers: #plasma, #vdg, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16045