qt history tab: fix double-clicking inside collapsible items

Swaps (both reverse and forward) are displayed in the list as a tree, which can be
expanded/collapsed. When expanded, double-clicking on any of the children (regardless
of child being LN or onchain), was interacting with the incorrect item (the first few
items in the top-level list).

Consider `self.tx_item_from_proxy_row(idx.row())`.
`idx.row()` is probably a small int, e.g. `0`, and `tx_item_from_proxy_row` is just
getting that item from the top-level list.

Note that the right-click>"View ..." context menu behaves correctly, so prior to
this commit it was inconsistent with double-clicking.

electrum/gui/qt/history_list.py

@@ -421,8 +421,8 @@ class HistoryModel(CustomModel, Logger):
             HistoryColumns.TXID: 'TXID',
         }[section]
 
-    def flags(self, idx):
-        extra_flags = Qt.NoItemFlags # type: Qt.ItemFlag
+    def flags(self, idx: QModelIndex) -> int:
+        extra_flags = Qt.NoItemFlags  # type: Qt.ItemFlag
         if idx.column() in self.view.editable_columns:
             extra_flags |= Qt.ItemIsEditable
         return super().flags(idx) | int(extra_flags)
@@ -658,11 +658,13 @@ class HistoryList(MyTreeView, AcceptFileDragDrop):
             assert False
 
     def mouseDoubleClickEvent(self, event: QMouseEvent):
-        idx = self.indexAt(event.pos())
+        org_idx: QModelIndex = self.indexAt(event.pos())
+        idx = self.proxy.mapToSource(org_idx)
         if not idx.isValid():
+            # can happen e.g. before list is populated for the first time
             return
-        tx_item = self.tx_item_from_proxy_row(idx.row())
-        if self.hm.flags(self.model().mapToSource(idx)) & Qt.ItemIsEditable:
+        tx_item = idx.internalPointer().get_data()
+        if self.hm.flags(idx) & Qt.ItemIsEditable:
             super().mouseDoubleClickEvent(event)
         else:
             if tx_item.get('lightning'):