v0.6.3

Back no longer walks through earlier packages

@missing-elements/h5p-offline-player 0.6.3.

When a page played one package after another in the same element, each switch added two entries
to the browser's history, and emptying the player added one more. Back then took the learner
through the earlier packages, loading each one again, before it left the page. A single-page app
lost its own steps too: after going back, the next switch or clear wiped out what Forward would
have returned to.

The element now moves its frame from one package to the next in place, so loading, switching and
emptying the player leave the page's history as it was. A page that plays one package and stays
on it was never affected, and neither are the embed page and the verifier, which load one package
per page. Inside the element's shadow root, the frame's src attribute now names the first package
the element loaded, not the current one; pkgId and the ready event say what is playing.

Nothing else changes, and the runtime stays at 0.1.2. On a CDN, update the player's URL to
h5p-offline-player@0.6.3; the runtime's stays at h5p-runtime@0.1.2.

Full Changelog: v0.6.2...v0.6.3