Material-ui-pickers: [Datepicker] - add ok and cancel button

[hadasmaimon]

• [x] The issue is present in the latest release.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

I don't know why I don't find how to add the ok cancel button (It works in the past)

Expected Behavior 🤔

expected to have the ok and cancel buttons.

Context 🔦

Your Environment 🌎

| Tech | Version |
| ----------- | ------- |
| Material-UI | v 4.0.0-alpha.7 |
| React | |
| Browser | |
| TypeScript | |
| etc. | |

[dmtrKovalenko]

There is no way to display dialog buttons on the desktop with <DatePicker />.
The only option is to use MobileDatePicker.

[oliviertassinari]

A couple of buttons and there use cases that come to mind for the date picker:

• "OK" when we don't want the "autoOK" behavior
• "Today" bottom when we get lost
• "Clear" if we want to reset the value to null

That could potentially be useful. I don't know if it's something that should be built-in, or only provide a customization point for doing it.

[dmtrKovalenko]

We could think about some option to render anything after the calendar. It could be helpful for buttons or displaying related helper information for users. Something like renderBottomInfo.

For instance react-dates have the following prop

<DateRangePickerWrapper calendarInfoPosition="bottom" renderCalendarInfo={renderCalendarInfo} autoFocus />

http://airbnb.io/react-dates/?path=/story/drp-calendar-props--with-info-panel-bottom

[oliviertassinari]

A random thought on the possible API:

```jsx
renderPopup={(props) => {

OK

}}
/>

[hadasmaimon]

good thanks!

[Jesusz0r]

how can I override the 'OK' and 'Cancel' text from KeyboardDatePicker?

[hadasmaimon]

What do you want to do? do you want to change the ok text? if yes use the okLabel

okLabel='Done'