Introduce abi, and icon to top-level App objects

[sohkai]

We should expose some easy-to-use APIs for fetching commonly-accessed metadata from App objects:

• ABI
• Icon (port over the code we have in the client)?
• Human read-able name (not sure what App#name is returning at the moment—probably the aragonPM name?)

However, I'm not sure how useful the icon is over the general case, as developers may prefer their own assets / etc.

cc @andy-hook @bpierre

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I couldn't find another issue for this, but this may also be a good place to start a discussion re-thinking the current data available from the App class.

Things I'm not sure about:

• kernelAddress and Organization could probably be reduced to just Organization?
• registry, registryAddress, repoAddress, version may be better encapsulated in a Repo class