Add eth_watchToken method to provider.

[danfinlay]

Would allow sites to suggest new tokens for the user to track in our token tracker:

ethereum/EIPs#747

Could maybe namespace under wallet_ or something, if we wanted to logically separate it from blockchain interactions.

Would allow users to easily add tokens to watch, with logos and everything, without us carrying the burden of verifying token projects and their logos.

Would require a new confirmation view "Do you want to add this token to your wallet?", along with the details of the token, including the current account's balance.

[bdresser]

closing - this was handled in #4606 @danfinlay ?