Scoped storage support for downloads

[TommyTran732]

Describe your suggested feature

Right now, to choose a custom downloads folder, Kotatsu requires access to all files. It would be nicer if it can just use scoped storage and prompt for a directory instead.

In fact, the "default page save directory" setting already does this, so it's a bit strange "Downloads folder" doesn't do the same.

Acknowledgements

• [X] This is not a duplicate of an existing issue. Please look through the list of open issues before creating a new one.

[Koitharu]

ZipFile that used for downloading does not support working with scoped storage and can be used only within plain file api.