Tree: checkbox selection, items where 'selectable:false' don't have a check box

[nanafocus]

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Tree: checkbox selection, items where 'selectable:false' still have a check box #5359


**Current behavior**
Items that have 'selectable' as false in the tree don't have checkbox.

**Expected behavior**
Checkboxes should be displayed on items where 'selectable' is false.

**Minimal reproduction of the problem with instructions**
Create a tree with a selection mode as checkbox. Set the items to 'selectable' false, note you will not see the checkbox.

**What is the motivation / use case for changing the behavior?**
I want to see the checkbox when selectable is false because I use this property to disable the tree. I want to see the complete tree with the checkbox also when it is disable.

* **Angular version:** 6.0.8

* **PrimeNG version:** 6.0.0

[estenp]

This doesn't actually appear to be a bug, but rather an intentional change made by the team in the 6.0 release (selectable option added to *ngIf on the .ui-chkbox : 1e66a7c...master).

I guess the question is why was this change made and would the team be willing to accept a pull request adding the old disabled checkbox display back in, or at least possible adding an option to tree node for turning it's display on or off.

[KhaledGabr]

This doesn't really fix the issue.
Seletable Property should be used to show or hide the selection checkbox.

Please, add a new property called, disabled that would disable a checkbox. However, the selectable button needs to be reverted back like it was before.