Install pri file for qmake support & document it in metainfo.yaml

Summary:
Allows to use KFileMetaDataBaloo in qmake-based projects with
Qt += KFileMetaData

Reviewers: #build_system, lbeltrame

Reviewed By: lbeltrame

Subscribers: #frameworks

Tags: #frameworks, #baloo

Differential Revision: https://phabricator.kde.org/D12424