Enterprise: Add Tree readonly() method

[davidcarlsonberg]

Is your feature request related to a problem? Please describe.
A feature method to readonly the entire Tree.

Describe the solution you'd like
The assumed solution is the ability to readonly the entire Tree by readonlyifying each node.

Describe alternatives you've considered
An alternative solution could be readonlyifying the parent element in Tree and styling the nodes as readonly based on a parent class.
An additional method could be to readonlyify a single node.

Additional context
Parent issue: #561 .

[davidcarlsonberg]

Barring any disagreement, I'm closing this ticket as Tree is not editable by the user and therefore anything accomplished by a readonly state is accomplished by a disabled state. If, at some point, a user is able to edit nodes, perhaps then we can revisit to distinguish between readonly and disabled in the context of Tree.

[EdwardCoyle]

No argument here!