[Icons] Add InMemory registry to allow easier tests

[smnandre]

Will come handy to test the IconRenderer

[kbond]

As discussed in Slack, do we want to make this a configuration option?

Allowing this to be used as the only registry in your test environment could save a ton of http requests for apps that extensively use the on demand system.

[smnandre]

Yep that was on my mind.. But i rather go step by step there, as i'm sure i need the InMemoryRegistry to add feature + test the Renderer :)

You ok if we add the config/test thing in another PR ?

[kbond]

Could you show an example of how you intend this to be used by end users?

[kbond]

Thanks Simon.

[smnandre]

Could you show an example of how you intend this to be used by end users?

First example there (internal so that's not the answer you expected ... but it's a start)

#1606