[FileContentIndexer] Connect finished signal from extractor process

Summary:
Only emit the "finished" signal to any watchers when the indexing
is really finished. Also reset currentFile on finished.

Test Plan:
start dbus-monitor
touch some larger files
-> some time passes between started and finished signals

Reviewers: Frameworks, poboiko, ngraham, Baloo

Reviewed By: ngraham, Baloo

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D15994