Bump @version to 0.5.0 and date the CHANGELOG entry. Merged the
draft 0.5.1 content (canvas component, .fresco file format) into a
single 0.5.0 release — neither was on hex yet, so consolidating
keeps the version history linear instead of inventing a 0.5.1 that
nobody installed.

OpenSeadragon is gone; <Fresco.viewer> is now a hand-rolled
CSS-transform pan/zoom engine. <Fresco.canvas> is the new layered
scene primitive (N images at absolute canvas-pixel coords plus an
open extensions map for peers like Etcher 0.3+).
<Fresco.scroll_strip> is unchanged.