Summary:
RecentUsageModel can list all aps and docs,
only aps or only docs. make possible to control that from QML

Test Plan: the api works, all existing code has the same behavior as before

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27782