Update Calendar markup for a11y

[jacobdevera]

Description

The markup should be updated to reflect changes made in fundamental-styles.
SAP/fundamental-styles#410

• Convert month/year selector ul into table with role=“grid”
• Currently selected date: <td>: aria-selected=“true” and <span> role "button"

Additional changes:

• Month/year header: aria-live=“polite” (to announce month/year changes)
• Navigation buttons: Needs localizable aria-label (e.g. “Next Year”, "Next Month")
• Selectable dates: aria-label should be used to read out full date
• Disabled dates: <td>: aria-disabled="true"

[jacobdevera]

fixed by #858