[Timeline] Use categorized logging

Summary:
Set all messages in regular execution paths to Debug (off by default),
and keep the only error message (parsing error) as Warning.

Test Plan:
make && ctest
entering e.g. "Last Month" from Places no longer fills the journal (3 log
lines per day, i.e. ~90 lines).

Reviewers: Baloo, Frameworks, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16044