AppBar Toolbar customization using children

[alanpoulain]

Is your feature request related to a problem? Please describe.
If you want to add elements to the Toolbar of the AppBar, you need to copy all the AppBar into your custom one.

Describe the solution you'd like
Why not allowing children to be passed to the Toolbar? Maybe between Typography and LoadingIndicator.

Describe alternatives you've considered
It could also be a render prop for the Toolbar.

Additional context
Example:

import React from 'react'
import { AppBar } from 'react-admin'

export default props =>
  <AppBar {...props}>
    <!-- Toolbar components here -->
  </AppBar>

[fzaninotto]

Fixed by #2777