+1
I also came here looking for such a feature!
EDIT: while we wait, I discovered a "shortcut".
Well, it isn't really, because you still have to close all the "Do you want to save, etc", but if you exit the app and close all dialogs except the last one (you can do this very fast, even before the program actually displays the dialog), then cancel and manually close the last file, you'll get a clean slate.
I work with LOTS of images at my job, and I'm a mess at keeping a clean workflow, so, instead of closing every file once I'm finish with it, I start to hoard open files, and this workaround helps a bit, even though, in extreme cases, terminating the program and starting fresh may be even faster, it depends on how quickly you computer fires up Photo or Designer.