Close each file after export

[gsokoll]

trafficstars

Current version opens each file for export, but does not close them. In projects with hundreds of files, this results large memory usage and slows performance. Proposed change closes each file immediately after exporting.

[ciscoski]

This PR should be merged. It is really useful.

[o-gent]

integrated the closing change and the file name issue here https://github.com/o-gent/Project-Archiver