Sweep keys: don't default to hidden address

The issue is electrum uses None as the current account
to mean the default account.  This just gave rise to
all addresses being listed including in hidden accounts.

Really "None" as used in main_window.py and history_widget.py
means "a single unspecified account" not "all accounts".

So implement the idea of a default account, and don't let
a hidden account be the default one.

lib/wallet.py

@@ -666,10 +666,13 @@ class Abstract_Wallet(PrintError):
         amount = max(0, sendable - fee)
         return amount, fee
 
+    def default_account_id(self):
+        return self.accounts.keys()[0]
+
     def get_account_addresses(self, acc_id, include_change=True):
         if acc_id is None:
-            addr_list = self.addresses(include_change)
-        elif acc_id in self.accounts:
+            acc_id = self.default_account_id()
+        if acc_id in self.accounts:
             acc = self.accounts[acc_id]
             addr_list = acc.get_addresses(0)
             if include_change:
@@ -1690,6 +1693,11 @@ class BIP32_HD_Wallet(BIP32_RD_Wallet):
                 set(['%d' % n for n in range(len(self.accounts))]))
         return len(self.accounts)
 
+    def default_account_id(self):
+        visible_acc_ids = [acc_id for acc_id in self.accounts.keys()
+                            if self.show_account(acc_id)]
+        return visible_acc_ids[0]
+
     def show_account(self, account_id):
         return self.account_is_used(account_id) or account_id in self.labels