[FileWatch] Fix watch updates on config changes

Summary:
Track the current list of include/exclude folders, and on a config change
add/remove watches for the differences. Previously, when an include
folder was removed the corresponding watches were never removed.

Depends on D29190, D29189

Test Plan: ctest -R filewatchtest

Reviewers: Baloo, ngraham, meven

Reviewed By: meven

Subscribers: meven, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D29191