Focus *picker inputs on tap

[IsaiahPacheco]

Problem description

For iOS tablet and phones (safari and chrome) I found that if you focus on a form "TextField" input, before tapping on or a opening a DatePicker or TimePicker, the focus will persist on the last input. To fix this on iOS:

I added the "onTouchTap" property to the *Picker

onTouchTap={this.excuseKeyboard}

and implemented component method (bound to component)

    excuseKeyboard(event) {
        event.target.focus();
    }

Without this the picker will open with keyboard open and I think would be more useable out of the box with this done automatically (only tested on iOS).

Versions

• Material-UI: 0.17.0 - 0.16.x
• React: 15.x
• Browser: iOS 10.x, mobile chrome and safari (ipad/iphone7/iphone6plus)

[oliviertassinari]

Closing for #4787