1.0.8: Let a screenshot name a dark counterpart for the panel's two appearances

A screenshot's whole point is showing what the panel will look like, and the
panel now has two looks. "screenshots" entries may declare a "dark" path
alongside "path", validated the same way: relative, inside the plugin, a
real image type. Also documents and tests the screenshots/light/ and
screenshots/dark/ pairing the panel reads when a manifest says nothing.