paper-checkbox should have an indeterminate state

[KyLeggiero]

For lists with a "Select All" checkbox, or for displays like this compatibility matrix, an in-between (commonly called indeterminate) state is necessary. Microsoft gives a great example of how this should happen:

I propose the following addition:

<paper-checkbox indeterminate> label</paper-checkbox>

Where this will always give the checkbox an indeterminate state. This might override the checked state, so the following:

<paper-checkbox indeterminate> label</paper-checkbox>
<paper-checkbox checked indeterminate> label</paper-checkbox>
<paper-checkbox indeterminate checked> label</paper-checkbox>

would all render exactly the same way.

[andreleblanc-wf]

@BenLeggiero you should file this against the paper-checkbox repo.

https://github.com/PolymerElements/paper-checkbox/issues

[KyLeggiero]

@andreleblanc-wf alright, resubmitted as paper-checkbox issue 33