Make sure only directories are added to the inotify watcher

Summary:
Although watchFolder is only called for directories, there is a small
time frame where the just added directory has been deleted and a file
with the same path has been created. This race condition can be avoided
by adding the KInotify::FlagOnlyDir filter flag.

This has been documented also in e.g. https://lwn.net/Articles/605128/.

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18776