Add indeterminate state

[grokys]

Paper checkbox should provide an indeterminate state, as does the standard checkbox. However it would be useful if this indeterminate state could be set from markup (it can only be accessed via script with the standard element).

[marcysutton]

We have a similar issue open for Angular Material, there actually isn't an indeterminate state in the published design spec: angular/material#1912

[KyNorthstar]

+1 this is absolutely necessary for "Select All" checkboxes. When some, but not all nor none, of the other boxes are selected, the top checkbox should be indeterminate, like:

☑ Section A
  ☑ option 1
  ☑ option 2
  ☑ option 3
⧅ Section B
  ☑ option 4
  ☐ option 5
  ☑ option 6
☐ Section C
  ☐ option 7
  ☐ option 8
  ☐ option 9

[grokys]

@BenLeggiero yes this is what I need it for.

[JulienOrain]

+1