Add support for making an item "hidden" within the layout.

[sdedios]

It would be very useful to have a Grid item be hidden based on a breakpoint. Hidden would be the equivalent of setting display: none on the column / row within the parent container. I think we need to use the property name visible because hidden is already reserved by HTML.

So for example:

<Grid container direction="row">
    <Grid item visible={{xs: false, md: true}}>This only appears on desktop</Grid>
    <Grid item>This is always shown</Grid>
    <Grid item visible={{xs: true, md: false}}>This only appears on mobile / tablet</Grid>
</Grid>