start to store the modification time for files in the database
Summary:
start to store the modification time for files in the database
this allows to scan files again only if they have a modification time greater than the one stored in database
Test Plan: tests are OK
Reviewers: Elisa, astippich
Subscribers: astippich
Maniphest Tasks: T7603
Differential Revision: https://phabricator.kde.org/D12622