INPUT_SELECTOR limits use of custom buttons

[bennypowers]

Consider:

<details id="update-dialog" hidden>
  <summary>New Version</summary>
  <details-dialog>
    <h1>New Version Available!</h1>
    <p>Reload the Page?</p>
    <mwc-button id="dialog-reload">OK</mwc-button>
    <mwc-button data-close-dialog>Cancel</mwc-button>
  </details-dialog>
</details>

This will fail entirely, since the requisite inputs are not found when the element boots up.

By adding mwc-button to the INPUT_SELECTOR, expected behaviour is restored.

I recommend adding a special attribute to the list, like so:

<details id="update-dialog" hidden>
  <summary>New Version</summary>
  <details-dialog>
    <h1>New Version Available!</h1>
    <p>Reload the Page?</p>
    <mwc-button data-dialog-input id="dialog-reload">OK</mwc-button>
    <mwc-button data-dialog-input data-close-dialog>Cancel</mwc-button>
  </details-dialog>
</details>

[muan]

Hi! Thanks for the issue. Just to give you an update- we are looking into this and want to come up with a solution that can best ensure these elements are indeed focusable to guard against human errors. We are going to look into how we might be able to work better with delegatesFocus shadow roots (in <mwc-button>'s case).