Use modal sheet instead of dismissable ImageGallery

[Cierra-Runis]

Since dismissable ImageGallery doesn't pushed to navigation, we can't use Android system back gesture to dismiss ImageGallery.
By using modal sheet, it can pop with back gesture and remove code that managing states of ImageGallery.

[demchenkoalex]

is it okay to include navigation/modals inside of the library? will it work nicely with any flutter application?

[Cierra-Runis]

Most of flutter projects are using MaterialApp, which need import material library, and showModalBottomSheet method is include in it.

If showModalBottomSheet method is not good for you, just use Navigator.push because Navigator must exist, or give an optional function for developer to handle it.

[demchenkoalex]

Well bottom sheet is not really suitable for the image preview, is it? Never ever saw image preview in a bottom sheet :)

I think I will just add some callbacks for library users to manage this, cause I don't feel super comfortable with navigating from the UI library. The image preview library is also kinda meh so I will allow for people to use their implementations.

[demchenkoalex]

image preview is dropped in v2 #646 due to the quality of the package used. Maybe eventually I will write yet another package for image preview specifically, but probably not soon.