AssetRegistry is not compatible with JSON v1

[hhaensel]

I've brushed up my old PR #14 to make AssetRegistry compatible with the latest JSON version.
The other optimisations seem still useful.

[hhaensel]

This is also a prerequisite to bump the JSON compat of WebIO and PlotlyJS

[hhaensel]

Are you planning a new release or are some more adjustments needed?

[hhaensel]

@shashi @twavv who could trigger a new release?