force-finish canberra notifications on close()

Summary:
`KNotification::close()` causes the manager to close the plugin for the
notification and after that KNotification will call deleteLater() on
itself. In the canberra variant of NotifyByAudio we handled this by calling
ca_context_cancel to abort playback of the audio. This ultimately would
still cause a finishCallback once the playback actually cancelled. The
callback does arrive in an undefined amount of loop cycles later though.
Put together this allowed for timing issues where deleteLater would run
before the finishCallback arrived, giving finishCallback the risk of
accessing a KNotification object past its lifetime and segfaulting.

To prevent this from happening we'll finishNotification in the plugin's
close(). This drops the notification out of the mapping hashes and tells
the manager that we are done. finishCallback now returns immediately if it
cannot find a mapping for a notification (i.e. it was close()d already).

CHANGELOG: Fixed a crash caused by bad lifetime management of canberra-based audio notification
BUG: 398695

Test Plan: added qdebugs. without patch close() and thus deleteLater() happens before finishCallback() but the callback still does its thing. with patch finishCallback is noop.

Reviewers: broulik, jtamate

Reviewed By: broulik, jtamate

Subscribers: kde-frameworks-devel, jtamate

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15638