api: add dc_replace_webxdc()

[link2xt]

This is an alternative to #4532.

The idea is to add a new JavaScript API window.webxdc.replaceWebxdc allowing the WebXDC app to replace itself. This will allow the apps to upgrade themselves by requesting a base64-encoded blob from a bot.

This will also allow the apps to modify themselves by repackaging all the files with necessary modifications (e.g. updated configuration). E.g. a TiddlyWiki port may replace itself in-place instead of saving a new copy of self to chat each time.

[link2xt]

It is actually ready, but needs a JSON-RPC API as well.

[link2xt]

The reason it is in the draft mode currently is because there is no decision on https://github.com/webxdc/website/issues/61