Add frontend support for displaying txs.

This change adds a new websocket extension command,
listalltransaction, which works just like listtransactions except it
does not take the count or from optional args, and will return an
array of all transaction details.  Notifications for newly-added
transactions are now sent to frontends as well, using the newtx
notification.

No support for updating tx details or removing failed txs is
implemented yet, and will be when cleanly failing a tx send is
implemented later.