Add NMF option to decomposition menu and toolbar

[jat255]

This PR adds an "NMF" button to the toolbar and decomposition menu that sits alongside the PCA and BSS buttons. I was a little unsure of how to implement it in some places, so I'd be happy to hear any reviews. I mostly copied the BSS code, since it has to do the same thing (display scree to user, get a click, and then perform the decomposition). It seems to work in my testing, but comments are welcome.