Group swap transactions in Qt history (fixes #6237)

- use tree structure of QTreeView
 - grouped items have a 'group_id' field
 - rename 'Normal' swap as 'Forward'

electrum/util.py

@@ -177,6 +177,9 @@ class Satoshis(object):
     def __ne__(self, other):
         return not (self == other)
 
+    def __add__(self, other):
+        return Satoshis(self.value + other.value)
+
 
 # note: this is not a NamedTuple as then its json encoding cannot be customized
 class Fiat(object):
@@ -216,6 +219,10 @@ class Fiat(object):
     def __ne__(self, other):
         return not (self == other)
 
+    def __add__(self, other):
+        assert self.ccy == other.ccy
+        return Fiat(self.value + other.value, self.ccy)
+
 
 class MyEncoder(json.JSONEncoder):
     def default(self, obj):