[FIX] web: x2many list: edit and long click 'Add a line'

Assume an x2many editable list where the user edits a record (input
field), and then clicks on 'Add a line' (and keeps it pressed for a
while, before releasing the click).

When the 'mousedown' event occurs, the 'change' event is triggered
on the edited field, and (once potential onchanges have been
applied), the list is updated, i.e. all rows except for the one
being edited a re-rendered.

However, the 'Add a line' handler listens to the 'click' event,
which never occurs if we release the click after the rows have been
re-rendered.

This is an issue for every rows, not only 'Add a line', but for
data rows, the issue is already present in former versions, and is
not that easy to fix). The 'Add a line' case is a consequence of
the recent work on list views (see 234d92c4), so we can fix it
independently.

Task 1915702