Move struct FileMetaData into separate file

Summary:
Move xliffextractor.* and poextractor.* into src/metadata/

These files are not specific to project/ , they are only about
extraction of metadata into struct FileMetaData.

Update includes

projectmodel: Move FileMetaData operator overloads into filemetadata.*

Test Plan: it compiles

Reviewers: sdepiets

Reviewed By: sdepiets

Differential Revision: https://phabricator.kde.org/D21753