Remove ability to disable selection propagation

[mellis481]

As a maintainer, I'm logging this "feature request" because it's something my team plans on executing in the foreseeable future, but I want there to be visibility to the consumers of this library. Please provide feedback if you have any regarding this issue.

Problem Statement
Disabling selection propagation upward and/or downward creates serious usability and accessibility issues.

Core Principles
There are two principles for a tree component that must be upheld with regard to parent nodes to ensure good usability and accessibility. A parent node should:

1. always correctly inform the checked status of its children.
2. provide a convenient means for selecting/unselected children.

Allowing selection propagation to be disabled in either direction goes against both of those principles.

Details
The usability and a11y issues are most easily seen when selection propagation is disabled in either direction and a parent node is collapsed. In this first example, if "Fruits" was collapsed, a user would believe no children were selected even though one or even all could be.

In this second example, if "Fruits" was collapsed, a user would believe all children would be selected even though none are.

From an accessibility standpoint, looking again at the first example, if a screen reader user focuses the "Fruits" checkbox, the screen reader would announced "Fruits not checked collapsed 1 of 3 level 1". This would have the user think "Since 'Fruits' is collapsed, that must mean it has children, but since it was announced as 'not checked', neither 'Fruits' nor any children are checked."

For these reasons, the ability to disable selection propagation should be removed.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue was closed automatically since it was marked as stale because it has not had recent activity. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue was closed automatically since it was marked as stale because it has not had recent activity. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue was closed automatically since it was marked as stale because it has not had recent activity. Thank you for your contributions.