p-calendar: tab key should focus the next tabbable element

[pfms84]

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Current behavior

When using a non-inline p-calendar, the tab key focus the first anchor to switch months and then closes the overlay, leaving the application without any focused element.

Expected behavior

The tab key should focus the next tabbable element

Minimal reproduction of the problem with instructions

Use a p-calendar
Focus the input to show the calendar
Press the tab key

• Angular version: 6.1.2

• PrimeNG version: 6.1.2

[Mysame]

Duplicates #6270

[bappana]

The issue still exists for IE11 with primeng version: 7.1.3

[cagataycivici]

Dup of 6270