port the potd kded to kdirwatch + lazy init consumer/engine

BLOCKED by the kded not being properly licensed !3 (comment 70527)

kdirwatch is a single inotify instance behind the scenes that is shared across all kdirwatch objects, so make use of the fact that kded will have a kdirwatch to monitor sycoca changes. this effectively reduces the inotify use by one, and that is important because there is a user-wide limit on how many instances may be held. holding more than one instance where it's not strictly necessary is a waste of resources.

also reduce memory footprint by lazy initializing the engine stuff until we actually know that potd is used