Support screens with different DPI

By changing the subcell-size variable, like GTK sugar-artwork does,
and outputting different CSS files. In particular, output CSS files
for 96dpi and 200dpi screens. In GTK they were named sugar-72.css and
sugar-100.css, but seems better to set more specific names:
sugar-96dpi.css and sugar-200dpi.css .
Amend the change in graphics README