Use propertymap to store properties in Baloo::Result

Summary:
Storing properties directly in a PropertyMap is more efficient (no
temporary conversion of property number to string, lookup with integer
key), and it also guarantees to have the same data as the extractor used
by BalooWigets, as long as (de)serialization to the Json format stored in
the DB is lossless.

Depends on D19087

Test Plan: ctest

Reviewers: Baloo, Frameworks, ngraham, poboiko, astippich

Reviewed By: Baloo, ngraham, astippich

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D19088