* Alternate fix for inconsistent list template syntax
First attempt, which fails on the ListWidget/WithMissingTemplate test.
* Make WithMissingTemplate test pass, inefficiently
Unfortunately, this ends up being very inefficient, because the
clone-and-mutate logic is repeated for every list item. Not ideal.
* More efficient way to do it
This also makes the failing test pass, but far more efficiently.
* Improve performance of list template discovery
Since parse tree nodes never change after widget creation (whereas
attribute values *can* change), we can safely search for the explicit
list templtaes only once, at widget creation time. This saves time as
the search doesn't have to be done on each re-render, and also allows us
to safely do a clone-and-mutate step to extract the list widget's body
(if any) without any `$list-empty` or other items. That, in turn, allows
using the list widget's body as the template even if `$list-empty` is
specified inside the widget body.
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