template-dialog-tree -- Create a tree inside template dialog

[deyan]

Replace the existing template dialog with a collapsable tree.

• The tree should have three state checkboxes
  • Selected state
  • Not selected state
  • Gray-out state (some of the children are selected)
• Selecting a child of empty parent should make the parent grayed out (if there are other unselected checkboxes) or selected (if all of the children are selected).
• Selecting/deselecting a parent selects/deselects all of its children
• The root and the first level should be seen on opening of the dialog
• Opened nodes and states of the nodes should be selectable by developers