[RFR] Convert AppBar to use useDispatch, useSelector, useWidth

[jaytula]

Convert AppBar to use useDispatch, useSelector, useWidth.

The useWidth was taken from material-ui documentation example: https://material-ui.com/components/use-media-query/#migrating-from-withwidth

Maybe useWidth should be defined in ra-core/src whereby it can be exported for other components that need it.

Done by request in #3582

[djhi]

Sorry about that, you should use the useMediaQuery hook:

const isXSmall = useMediaQuery(theme => theme.breakpoints.down('xs'));

And then:

variant={isXSmall ? 'regular' : 'dense'}

[djhi]

Nice! Thanks :)