Add `FundingTxBroadcastSafe` event

Add `FundingTxBroadcastSafe` event

The `FundingTxBroadcastSafe` event indicates that we have received
`funding_signed` message from our counterparty and that you should
broadcast the funding transaction.

This event is only emitted if upon generating the funding transaction
you call `ChannelManager::unsafe_manual_funding_transaction_generated`
that will emit this event instead of `ChannelPending` event.

`ChannelManager::unsafe_manual_funding_transaction_generated` wont check
if the funding transaction is signed, those its unsafe. It is manual
because you are responsibile on broadcasting the transaction once the
event is received.