Add 48 pixel notebook icon

This has been sitting uncommitted for a while. We have this icon at 16, 24, 32, 64, 128, 256 and 512 pixel resolutions, but not at 48 pixel, which seems to be a default of some kind for Linux ("Minimally you should install a 48x48 icon in the hicolor theme." - [xdg spec](http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#install_icons)). I generated this for nbopen.