Support hiding/showing tabs instead of destroying/creating

[jelbourn]

This request for tabs comes up some some frequency. MatTabGroup uses ngIf to show the current tab, meaning that tab content is torn down when switching between tabs. Sometimes people want to preserve uncommitted state across tab changes (e.g. #17034 and #17391), which this prevents.

[crisbeto]

I think this should be covered by #17391.

[jelbourn]

@crisbeto let's track it here since this is the root of the issue.