* Fix bug with checkbox widget and `list` field
The `list` field is stored as a list and frozen against modifications,
and getFieldList() returns it directly without creating a copy. So
before we modify it, we need to make a copy so we're not modifying a
frozen list. This bug doesn't manifest with custom fields, which are
stored as strings, only with the built-in `list` field.
* Fix checkboxes referencing non-existent tiddlers
This fixes the "tiddler is undefined" error when a checkbox's listField
property references a tiddler that doesn't (yet) exist.
* Better logic for checkbox listField handling
If the field contains an array, then it's almost certainly referenced
elsewhere and needs a defensive copy made. If it contained a string,
then it's safe to modify without making a defensive copy.
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