Currently fetch() would invoke replace() which would block on QFile.exists()
while waiting on filesystem. If this is remote filesystem or just very slow
it can cause significant (ie. even several minutes) delay of Plasma startup time.

Improve this by simply doing IO in a seperate thread.

This allows to forget many resources in single call
and avoiding unnecessary activity fetching

Task tracked at:
teams/frameworks-devs/kf6-workboard#2

So it doesn't pollute the cmake output when building with BUILD_TESTING=OFF

Instead of /usr/include/KF5/kactivitiesstats_version.h.

For more details see:
kservice!79

GIT_SILENT

Task: https://phabricator.kde.org/T15109

Reduce the use of boost in KDE Frameworks

Other KF repos have it, and it's useful

NO_CHANGELOG

The current solution with configure_file gave me a .pc file with
incorrect values, e.g.:

$ pkg-config --cflags libKActivitiesStats
-Iinclude -DQT_CORE_LIB -I/usr/x86_64-pc-linux-gnu/include/qt5/QtCore -I/usr/x86_64-pc-linux-gnu/include/qt5
$ pkg-config --libs libKActivitiesStats
-Llib -lKF5ActivitiesStats -lQt5Core

Using ecm_generate_pkgconfig_file fixes that and should also be nicer
to maintain.