Use KDirWatch removeDir/addDir instead of stopDirScan/restartDirScan

Summary:
This fixes notifications happening against our will given that inotify
is async.

Test Plan: copying/deleting files in dolphin still updates the view

Reviewers: aacid

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5856