Tri-State Checkbox

[Gu Yi, He]

TriStateCheckBox? is check box that can be checked, unchecked, or of an indeterminate state.
Most often, TriStateCheckBox? can be found in a tree.
TriStateCheckBox? behavior:

1. When no children elements are selected, the parent element is also unselected.
   
2. When all the children elements are selected, the parent element is selected.
   
3. When the child elements are both selected and unselected, the parent element is in an indeterminate state. Selecting a parent indeterminate state check box selects all items. Selecting the parent check box a third time returns the check box to the indeterminate state.
   

API:
TriStateCheckBox? almost shares the same api of diji.form.CheckBox?. Here's a list of difference:
Control the state of a TriStateCheckBox? :
.set("checked", true)
.set("checked", false)
.set("checked", "mixed") (dijit.form.CheckBox? does not have this state).

Set value for a TriStateCheckBox:
.set("value", foo) same as CheckBox?

Set values for a TriStateCheckBox:
.set("values", [foo, bar]) foo/bar is the value to be submitted when the state of TriStateCheckBox? is checked/mixed.

Set states cycle for a TriStateCheckBox::
.set("states", [true, false])

[bill]

Dup of #7513 and #7514.