Delete from missing_files when deleting file.
Fix setting file_hash in kdenlive xml with proxies.
Display revision comment on history page.
Fix error in MltXmlWriter.
Add Upload New Version button to view projec file page.

Now, you can upload modified projects, and those revisions go into the
history table. Even though you can not yet view the actual changes
within the project file, it does reflect a default change comment of "Upload
New Version" (same as button label).
Improve appearance of jQuery UI buttons.