[SearchStore] Handle double values in property queries

Summary:
Allows to e.g. query Photo exposure times, aperture etc.

Depends on D28400

Test Plan:
baloosearch 'photoExposureTime>0.5'
baloosearch 'photoDateTimeOriginal>="2019-12-21 10:00:00"' \

AND 'photoApertureValue>4.0'

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28401