rename and export newBatchTime signal in filecontentindexer

Summary:
Applications can subscribe for new files being indexed
by baloo via the dbus interface of fileindexer. However,
currently only the start and finishedIndexing signals are
exported. The latter does only indicate that the file has
been indexed, but not yet committed to the database.
Rename the newBatchTime to better reflect that the
batch has been committed to the database and export
it so that it is possible to know when the transaction
has been committed and thus is safe to query the
database for metadata.

Reviewers: Baloo, bruns

Reviewed By: Baloo, bruns

Subscribers: mgallien, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D28682