[DataGrid] Fix Popper z-index

[m4theushw]

One chunk of #844

[oliviertassinari]

The updated reproduction with the latest commit of the PR: https://codesandbox.io/s/affectionate-tereshkova-tphdf?file=/package.json.

[TodBob]

The thing is, probably not related with this fix. Filter is not working. You can't type anything in value input.
https://codesandbox.io/s/affectionate-tereshkova-tphdf?file=/package.json.

In MUI 4.11.1 if I test demo from docs. Filter is not working, value input seems disabled or something.

[m4theushw]

@TomasKomprs That is because the TrapFocus keeps moving the focus to the Dialog. It's fixed in v5. To work with v4 see this comment.

[oliviertassinari]

It seems that we didn't yet get any volunteers to take ownership of the problem by the action of merging this PR. It looks great, @m4theushw do you want to do it? :)