H5PExport::createExportFile() performance issues

[catch56]

H5PExport::createExportFile() can cause significant performance issues on network file systems - I've seen it taking 40-60s when saving a column activity using the H5P Drupal module. The same activity takes around 750ms on my local install which is still quite significant.

The main issue is with how dependencies are handled, currently looks like:

• loops over every dependency
• copies all of the files from each directory to a folder in /tmp
• reads those files back out of the /tmp folder into a zip.
• deletes the files in the tmp directory

Instead, could it:

• loop over every dependency, build up a list of files in their current locations
• add those files to the Zip