[QML Monitor] Show remaining time as soon as possible

Summary:
The update timer was started when the content indexing started. If a large
file is encountered during the first second, the estimate could stay
at "Estimating" for a considerable amount of time.

If baloo_file has already indexed some files, an estimate is available
immediately. Replace the QElapsedTimer with a QDeadlineTimer and
initialize it to expired, so the first file triggers an estimate update.

Test Plan:
make sure baloo_has indexed enough files for an estimate (5 batches)
touch a big file -> estimate is shown immediately

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28956