notificationmanager: Don't store KFilePlacesModel in a weak pointer

KFilePlacesModel is used to look up "pretty names" for jobs. It is quite
expensive to construct. The current code tries to ensure there is a
single instance around when there are jobs active, however because it
uses a weak pointer for storage, when there is no job around, the model
gets destroyed and we recreate it when there's a job again. To avoid
this, just store it in a "strong" pointer so we create it once and it
stays around.