Add kio recentlyused:/ to access KActivityStats data

Summary:
Prior to finish D7446, create a better recently used documents feature
See https://phabricator.kde.org/D7446#485917 for motivation.

Test Plan:
compile & install
kdeinit5
kioclient5 ls recentlyused:/?limit=100
Or in dolphin

Current filter options :
agent, activity, type(mimetype), url(path fitering)
order and limit

Examples (to use with kioclient5 or see in dolphin) :
recentlyused:/?type=inode/directory
recentlyused:/?type=video/*,audio/*
recentlyused:/?path=/home/meven/kde/src/*&type=text/plain
recentlyused:/?order=HighScoredFirst
recentlyused:/?agent=gwenview

See recentlyused.h for more details.

Reviewers: ivan, Frameworks, ngraham, dfaure

Reviewed By: ivan, ngraham, dfaure

Subscribers: broulik, dhaumann, elvisangelaccio, kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D22144