Automated response: Cannot edit root download category due to required parent field

Editing the home download category results in a problem: it asks for a parent category, which is required, but a root category has no parent.

This patch fixes this; the parent field is not visible when editing the root download category. And edit_download_category now properly handles root (null) parent ID.

A hotfix (a TAR of files to upload) has been uploaded to this issue. These files are made to the latest intra-version state (i.e. may roll in earlier fixes too if made to the same files) - so only upload files newer than what you have already. If there are files in a hot-fix that you don't have then they probably relate to addons that you don't have installed and should be skipped. Always take backups of files you are replacing or keep a copy of the manual installer for your version, and only apply fixes you need. These hotfixes are not necessarily reliable or well supported. Not sure how to extract TAR files to your Windows computer? Try 7-zip (http://www.7-zip.org/).