DO NOT MERGE: to be reviewed.
This feature request was born out of wanting the ability to disable (for example) a top level page from being selected, while still being able to select a child page. Using setFilterFunction() simply removes the node and its children.
Extra styling for disabled nodes
Disable ability to select a disabled node for TreeDropdownField
Disable hover CSS changes
Fixing merge conflict during rebase
Return a boolean for nodeIsDisabled()