Added unbound DisclosureGroup constructors and fixed animated updates

[fhasse95]

This PR adds support for using DisclosureGroup without an external isExpanded binding. When no binding is provided, DisclosureGroup now manages its expansion state internally, similar to other controls like Picker.

Example:

DisclosureGroup("Title") {
    Text("Content")
}

It also fixes DisclosureGroup animations when used inside Form and List. As part of this, List now uses stable row and section keys for animated updates, fixing the visual artifacts that could appear when rows were inserted or removed. This improves not only expanding and collapsing DisclosureGroups, but also animated List updates in general, for example when rows are inserted or removed during an animated state change.

The behavior after the bugfix can be verified in the FormPlayground of the Skip Showcase app:

Before:

The DisclosureGroup animation was not fully consistent and could jump during expansion or collapse, which caused visual artifacts in the following list rows.

After:

After this fix, the animation is consistent: list rows are inserted correctly, the trailing disclosure indicator rotates as expected, and following rows move without visual artifacts.

Related PR:

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