[SearchStore] Allow querying exact matches for non-properties

Summary:
Exact matches were only possible for prefixed properties, e.g.
filename=test or tag=test, but content matches were always
prefix queries. Allow to control the query type, i.e. content=test
(exact) or content:test (prefix).

Test Plan:
$> echo "test" > doc1.txt
$> echo "testing" > doc2.txt
$> baloosearch content:test -> doc1, doc2
$> baloosearch content=test -> doc1
$> baloosearch content=testing -> doc2

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28924