add genesis hash getters

lib/wallets/crypto_currency/coins/bitcoin.dart

@@ -18,6 +18,18 @@ class Bitcoin extends Bip39HDCurrency {
     }
   }
 
+  @override
+  String get genesisHash {
+    switch (network) {
+      case CryptoCurrencyNetwork.main:
+        return "000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f";
+      case CryptoCurrencyNetwork.test:
+        return "000000000933ea01ad0ee984209779baaec3ced90fa3f408719526f8d77f4943";
+      default:
+        throw Exception("Unsupported network: $network");
+    }
+  }
+
   @override
   Amount get dustLimit => Amount(
         rawValue: BigInt.from(294),

lib/wallets/crypto_currency/coins/epiccash.dart

@@ -13,6 +13,11 @@ class Epiccash extends Bip39Currency {
     }
   }
 
+  @override
+  String get genesisHash {
+    return "not used in epiccash";
+  }
+
   @override
   // change this to change the number of confirms a tx needs in order to show as confirmed
   int get minConfirms => 3;

lib/wallets/crypto_currency/crypto_currency.dart

@@ -21,5 +21,8 @@ abstract class CryptoCurrency {
 
   int get minConfirms;
 
+  // TODO: [prio=low] could be handled differently as (at least) epiccash does not use this
+  String get genesisHash;
+
   bool validateAddress(String address);
 }