New TryEditFile event

The implementation is a bit hackish, but there don't seem to be any bugs.  Thanks to the new event, my Sound Test plugin will now play double-clicked sound files from the project tree (yay, less distractions!).  Read the diffs, some of my comments should be good for a laugh :D

Note that there are a lot of deletions, I tend to be somewhat aggressive when it comes to deleting old code, this helps spot bugs related to calling stale, outdated methods that would go unnoticed otherwise.