Transaction states: cancellation spike

[ryancreatescopy]

Closely related to #82

This issue pertains to the transaction state work we've been looking into.

We need to figure out:

• can we give users a short window after submitting a transaction to cancel it if they submitted the transaction in error? Similar to how you an undo sending an email in gmail.

It's better to help a small percentage of users rectify a mistake rather than ask all users to double and triple check their Txs

• Can we offer this functionality in the dApp and send it to the wallet? E.g. can a user press a button to cancel a transaction in dApp or must it come from an interaction from within the wallet?
• At what point can a user no longer cancel?
• Can a dApp detect when the Tx in question has been cancelled so we can communicate that information back to the user in the dApp interface?

Pictured below is:

• an example UI for our transaction states: can a user stop a transaction from here?
• the MetaMask UI once you cancel a started Tx (for reference)

I believe this is important. The window for cancelling a transaction will likely be small so if we can save users seconds by allowing them to do this from the dApp UI this should make life easier for them.