Send a plug-in event when file is moved

I use this in one of my plug-ins to notice when I've moved all the audio
files in an album from one directory to another, at which point I move
any associated non-album files to the new directory and delete the old
directory.
Document new plug-in event item_moved
Add Item instance to item_moved event

This brings this plug-in event in to line with similar events.
Merge pull request #335 from dsedivec/master

Send a plug-in event when file is moved