Allow to pass 'useRootNavigator'

[fhuonder]

Hi

Would be great when the property useRootNavigator of showDialog or showMaterialModalBottomSheet respectively could be passed into the widget.
I currently have an issue where the dialog uses the wrong navigator and I cannot change it because showDialog has useRootNavigator = true by default. Btw. showMaterialModalBottomSheet has useRootNavigator = false by default.

Regards,
Florian