[RFR] Automatically add id attribute on input if not specified and accessibility fixes

[djhi]

• Infer id from source prop and set the id and htmlFor props automatically
• Adds aria-label to buttons

This allows material-ui to bind labels to their inputs using the for attribute, simplifying e2e tests with libraries such as react-test-utils and providing better accessibility.

For example, using cypress-testing-library in cypress, with an input written as:

<TextInput source="first_name" />

We can now write tests like:

cy.getByLabelText('First Name').type('Gildas');

Before, for the same result, we would have written the input as:

<TextInput id="first_name" source="first_name" />

NOTE: it is still possible to override the default id inferred from the source prop

[fzaninotto]

Awesome! Although, strictly speaking, this is an evolution, so it should land in next rather than master. Can you rebase to the next branch?