add get_master_public_key to all wallets

gui/kivy/uix/ui_screens/status.kv

@@ -49,7 +49,7 @@ Popup:
                 TopLabel:
                     text: _('Master Public Key')
                 RefLabel:
-                    data: app.wallet.get_master_public_key()
+                    data: app.wallet.get_master_public_key() or 'None'
                     name: _('Master Public Key')
                 TopLabel:
                     id: seed_label

lib/wallet.py

@@ -231,6 +231,9 @@ class Abstract_Wallet(PrintError):
         self.use_encryption = use_encryption
         self.storage.put('use_encryption', use_encryption)
 
+    def get_master_public_key(self):
+        pass
+
     @profiler
     def load_transactions(self):
         self.txi = self.storage.get('txi', {})