Returning default account as coinbase + allow altering sender in signer

[tomusdrw]

Coinbase will now return first account for particular dapp (so you can either use web3.eth.accounts[0] or web3.eth.coinbase) to get default account.
We agreed with @jacogr that the UI will always set default account as first on the list (either using parity_setDappsAddresses(dapp, [accounts]) or parity_setNewDappsWhitelist([accounts]))

Additionally signer RPC now allows to override transaction sender. PR allowing to omit from in TransactionRequest will follow.

Related: #3797

[gavofyork]

web3.eth.coinbase doesn't really work for me as a standard API. it already means something else. i'm happy to support existing dapps which use it to mean "default user account" but i'd prefer not to crown it.

web3.eth.accounts[0] also doesn't fly especially well for me as it places a semantic onto the order of the list with is self-inconsistent. the first -> second item difference is crucial but then the second -> third (and all further next-door comparisons) are irrelevant. as my 2nd term university lecturer would have said, "it's a hack".

let's just have parity_defaultAccount (along with a change event to subscribe to) and be done with it.

[tomusdrw]

Added parity_defaultAccount RPC, subscriptions will come with #884