Move unrelated code from projectmodel.{cpp,h}

Summary:
projectmodel: Extract class UpdateStatsJob into separate source files
projectmodel: Move FileMetaData extraction function into class FileMetaData

Test Plan: it compiles

Reviewers: sdepiets

Reviewed By: sdepiets

Subscribers: #localization

Differential Revision: https://phabricator.kde.org/D21843