Window minimize icon

I've added an icon for use for when matplotlib windows are minimized and
chose an arbitrary icon (back.svg) to get it working on the GTK+
backend. Since the icon has to be resized to fit in the panel, and svg
are good at resizing, I used svg instead of the usual png format (png or
other formats could be used instead if required).

John, if you can find/create a suitable matplotlib icon just copy it to
images/matplotlib.svg and overwrite the existing icon file.