Store Transactions in a tview.List

[Chris Gianelloni]

Currently, we use a strings.Builder and create one big string to store our data. Let's improve this by creating a Tx struct and a []Tx slice. This will hold our full transaction info. For tview, we create a txList and display it within the main Flex.

Each loop (unless paused), we reconstruct the []Tx slice from the list of transactions in the mempool. If the user selects a transaction, we should display more information about it.