Add possibility to tap the widgets behind the ModalSheetPage

[AlinaStepanova]

In some cases there is a need to access and interact with widgets that are behind the ModalSheetPage content when the modal sheet is still being opened (eg. dragging the map or taping the button).
In order to achieve that the boolean flag allowTapBehindModalPage was added that is false by default.

[fujidaiti]

Hi @AlinaStepanova, thanks for your contribution.

when the modal sheet is still being opened (eg. dragging the map or taping the button).

I think persistent sheets are more appropriate for these cases. Are there any reasons to prefer modal sheets?

[AlinaStepanova]

Hi @fujidaiti, sorry for the late reply. In my case, I use NavigationSheet since I need to be able to handle nested navigation of multiple sheets. As far as I understood this is achieved with the modal sheets under the hood. I understand that this is rather a rare use case. In case if the same behaviour can be implemented using persistent sheets, please let me know.

[fujidaiti]

NavigationSheet can be persistent. See tutorial/imperative_navigation_sheet.dart, or tutorial/declarative_navigation_sheet.dart if you use Navigator 2.0 (such as go_router).