odr.chunks.loadAll(): load every part before a print #1281

Asked for by the iOS session (opendocument-app/OpenDocument.ios#219).

With HtmlConfig::lazy_loading, a print shows a part that has not loaded as empty space (docs/design/lazy-loading.md, open question "Print at level 3"). beforeprint cannot wait for scripts. So the iOS app loads everything itself before it prints: it calls odr.chunks.show on each [data-odr-chunk-src] chunk, then waits until no [data-odr-chunk-loading] is left.

Proposal: odr.chunks.loadAll() returns a Promise that resolves once every part has filled, or that rejects if a part fails to load. A host awaits it before a print. The Android evaluateJavascript cannot await a Promise, so a host bridge message, such as chunksLoaded, would serve the droid app.

Loading every part costs what level 3 avoids, so the API should be for an explicit print, not for every page load.