qt history tab: fix double-clicking inside collapsible items
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.

spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum