FEATURE: Disable specific tree nodes in TreeDropdownField

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()
Merge pull request #2037 from kinglozzer/feature-disable-treemultiselect-nodes

FEATURE: Disable specific tree nodes in TreeDropdownField