D9165: Lazy-instanciate and lazy-load KNotification plugins.

[Eike Hein]

hein added a subscriber: Frameworks.

REPOSITORY
  R289 KNotifications
REVISION DETAIL
  https://phabricator.kde.org/D9165

To: hein, #plasma, #frameworks, davidedmundson
Cc: #frameworks, plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
jensreuterberg, abetts, sebas, apol, mart