InputMask misses an onFocus output event

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Current behavior
Currently, only onBlur and onComplete exist. Additional onFocus event required.

Expected behavior
Since onBlur and onFocus are mutual, it would be very helpful to emit onFocus, too.

Minimal reproduction of the problem with instructions

What is the motivation / use case for changing the behavior?

• Angular version: 4.0.1

• PrimeNG version: 4.1.0-rc.2

• Browser: [all]

• Language: [all]