add an aria-label to avoid the dialog content being read twice

[vlukashov]

Without an aria-label screen readers (NVDA, JAWS, VoiceOver) read the dialog content twice: once as a title and another time as the content.

Depends on vaadin/vaadin-dialog#91
Fixes #29

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This change is 

[alvarezguille]

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bower.json, line 31 at r1 (raw file):

2.1.0-beta2

Let's not depend on new minor version unless a stable version is released before V11beta

[alvarezguille]

Reviewed 1 of 2 files at r1.
Reviewable status: 1 of 2 files reviewed, 1 unresolved discussion (waiting on @vlukashov)

[alexberazouski]

Reviewable status: 0 of 1 files reviewed, 2 unresolved discussions (waiting on @alvarezguille and @vlukashov)

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src/vaadin-confirm-dialog.html, line 227 at r2 (raw file):

        _getAriaLabel(header) {
          return header || 'confirmation';

There is an option to set a custom header element into slot. Should we support this case as well?

[alexberazouski]

Reviewed 1 of 2 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved

[alexberazouski]

Reviewable status: complete! all files reviewed, all discussions resolved