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/gui/qt/custom_model.py

@@ -0,0 +1,94 @@
+# loosely based on
+# http://trevorius.com/scrapbook/uncategorized/pyqt-custom-abstractitemmodel/
+
+from PyQt5 import QtCore, QtWidgets
+
+class CustomNode:
+
+    def __init__(self, model, data):
+        self.model = model
+        self._data = data
+        self._children = []
+        self._parent = None
+        self._row = 0
+
+    def get_data(self):
+        return self._data
+
+    def get_data_for_role(self, index, role):
+        # define in child class
+        raise NotImplementedError()
+
+    def childCount(self):
+        return len(self._children)
+
+    def child(self, row):
+        if row >= 0 and row < self.childCount():
+            return self._children[row]
+
+    def parent(self):
+        return self._parent
+
+    def row(self):
+        return self._row
+
+    def addChild(self, child):
+        child._parent = self
+        child._row = len(self._children)
+        self._children.append(child)
+
+
+
+class CustomModel(QtCore.QAbstractItemModel):
+
+    def __init__(self, parent, columncount):
+        QtCore.QAbstractItemModel.__init__(self, parent)
+        self._root = CustomNode(self, None)
+        self._columncount = columncount
+
+    def rowCount(self, index):
+        if index.isValid():
+            return index.internalPointer().childCount()
+        return self._root.childCount()
+
+    def columnCount(self, index):
+        return self._columncount
+
+    def addChild(self, node, _parent):
+        if not _parent or not _parent.isValid():
+            parent = self._root
+        else:
+            parent = _parent.internalPointer()
+        parent.addChild(self, node)
+
+    def index(self, row, column, _parent=None):
+        if not _parent or not _parent.isValid():
+            parent = self._root
+        else:
+            parent = _parent.internalPointer()
+
+        if not QtCore.QAbstractItemModel.hasIndex(self, row, column, _parent):
+            return QtCore.QModelIndex()
+
+        child = parent.child(row)
+        if child:
+            return QtCore.QAbstractItemModel.createIndex(self, row, column, child)
+        else:
+            return QtCore.QModelIndex()
+
+    def parent(self, index):
+        if index.isValid():
+            node = index.internalPointer()
+            if node:
+                p = node.parent()
+                if p:
+                    return QtCore.QAbstractItemModel.createIndex(self, p.row(), 0, p)
+            else:
+                return QtCore.QModelIndex()
+        return QtCore.QModelIndex()
+
+    def data(self, index, role):
+        if not index.isValid():
+            return None
+        node = index.internalPointer()
+        return node.get_data_for_role(index, role)