Payment proofs (#52)

- Ability to export and verify payment proofs

Some fixes:
- Migrated tx heights store from lmdb (also changed heights key from local id to slate_id to avoid conflict between wallets)
- Close address panel on wallet change

Reviewed-on: https://code.gri.mw/GUI/grim/pulls/52

src/node/config.rs

@@ -398,7 +398,6 @@ impl NodeConfig {
             .unwrap()
             .wallet_listener_url = id.to_string();
         w_config.save();
-        println!()
     }
 
     /// Get the amount of time in seconds to attempt to mine on a particular header.