Eventual connectivity, caching and local storage

[DavidMS73]

Analyzing Kalium, some improvements correspond to:

1. Save the monkey image. These images correspond to the main account, the secondary accounts, and the contacts.
2. Save the latest transactions. In case the connection is lost, it should also show a message indicating the last time this information was retrieved.
3. Save the amount of Banano coins. In case the connection is lost, it should also show a message stating the last time this information was saved since the user can execute more transactions from the web.

For these improvements plugins such as flutter_cache_manager and cached_network_image could be used.