Support new txDropped transaction state

[cmeisl]

Back-end now sends a txDropped when a transaction is no longer in the mempool and not replaced (speedup/cancel) or added to a block.

Note that this feature is not necessarily a global state as it depends in specific node mempools. It is possible to a tx to receive txConfirmed/txFailed after a txDropped.

Support includes default and custom notification messages, as well as custom notifications event handler.

[lnbc1QWFyb24]

@cmeisl I have a branch setup with code that implements this feature in this commit: 848da94

Off the top of my head I can't think of a way to test this functionality. Is there a way that we can get a transaction in to the txPool and then get it to be intentionally dropped?

Also to be clear, you are saying in the above issue that a transaction can get dropped from the txPool and then get back in to the txPool without any additional actions by the dapp/user/Assist? The above commit implements that assumption and doesn't remove the transaction from the internal transactionQueue so that Assist can handle any new status updates that come through for that transaction.

I am also not sure whether this should be an "error" styled notification or a "pending" styled notification. The above commit implements it as an "error" styled notification.