[RFR] Add useTranslate hook

[fzaninotto]

• Add useTranslate Hook
• Document it
• Test it
• Use it in simple example
• Use it in demo example
• upgrade ra-core to use it instead of withTranslate in function components
• upgrade ra-ui-material-ui to use it instead of withTranslate in function components
• use useTranslate in class components (requires translating these components to functions, to be done sometime in the future)

// in src/MyHelloButton.js
import React from 'react';
-import { translate } from 'react-admin';
+import { useTranslate } from 'react-admin';

-const MyHelloButton = ({ translate }) => {
+const MyHelloButton = () => {
+   const translate = useTranslate();
    return (
        <button>{translate('myroot.hello.world')}</button>
    );
}

-export default translate(MyHelloButton);
+export default MyHelloButton;

[fzaninotto]

Switching to RFR

[fzaninotto]

The diff looks much better with 'hide whitespace changes' turned on.