Tokens: implement token transfers

[izqui]

For transferable tokens, we should implement token transferability within the client:

Right now if you want to transfer tokens, you need to use a wallet, add the token and then transfer it. Alternatively, you can burn and assign tokens to another account, but it kind of misses the point of transferable tokens.

Token transfers would be done with an external intent (introduced aragon/aragon.js#328) to the token: token.transfer(to, amount).

[sohkai]

Right now if you want to transfer tokens, you need to use a wallet, add the token and then transfer it.

This has always been the way I've told people to do this, since it's kind of the point of your wallet. We've previously discussed if adding this is a good idea, and decided no, but agree that it could be nice to have now that we have external intents ready.