[Demos] Multiline Autocomplete

[RoiEXLab]

Hi, after migrating a project from 0.20.1 to the most recent version of material-ui I noticed a small number of minor issues with the (otherwise awesome) auto complete demo.

I decided to go with the react-autosuggest tutorial (the issue I'm about to describe will probably affect the other versions too, at least to some degree), because it's the most lightweight dependency of all 3 options.
It works, which is great but here are the issues (I haven't been able to fix myself so far) that I have noticed that occur once the internal TextFields multiline prop gets set to true, that I like to see fixed in the provided demos:

1. The MenuItem doesn't really render line breaks \n which makes sense and can be worked around by splitting the string on line breaks, and putting
   tags between the individual elements, but really should be supported by the demo.
2. When using arrow keys to select an entry and pressing enter, this works, but also inserts a newline in the generated text area and under certain circumstances re-opens the suggestions display when there are matching items.
3. Not really your fault, but moving the cursor inside a textarea using the arrow up and down is not possible when no suggestions are displayed, I'm assuming this is due to react-autosuggest consuming the event in order to select the suggestions.

Unrelated to that I also noticed that the Suggestions are rendered behind a dialog when the autocomplete component is inside a dialog (using popper). This should be fixable by setting the container prop of the Poopper to the dialog container instead of document.body.
As a workaround I used a zIndex of 1400 to display it in front of the 1300 dialog.

I know this is much to ask for, especially because you don't really control the external dependencies, so I'd be happy to help out on some things if you can nudge me in the right direction.