Generate invoice + smaller refactors

This PR adds the screen where invoices can be generated and also makes the following changes among smaller refactors:

Adds appbar to screens to have consistent titles and back buttons
Changes refresh button in WalletInfoContainer to info button that shows a popup with the node info that can be used to open a channel to the node
Refreshing can now be done by pulling down the body of the Lightning wallet home screen

kumulynja / ldk_node_flutter_demo