D26604: Check if there is an activatable service when notification service owner changes

[Nicolas Fella]

nicolasfella added a dependent revision: D26605: Remove fallback to 
KPassivePopup.

REPOSITORY
  R289 KNotifications

REVISION DETAIL
  https://phabricator.kde.org/D26604

To: nicolasfella, #plasma, broulik
Cc: kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns