Add NewFolderSave dialog for creating new folder

[xypwn]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

I am writing a project where I would like to save multiple files to a user-accessible location on Android without requiring a dialog for each saved file.

Is it possible to construct a solution with the existing API?

No

Describe the solution you'd like to see.

I'm not very familiar with Android APIs, but I think the relevant part of the code is found in GoNativeActivity.java. The current issue is when a folder is opened, users are only allowed to read from this folder. This is what we would expect when using NewFolderOpen, but we could think about adding a function like NewFolderOpenForWriting (we can discuss the exact name). This new function would allow apps to create their own folders where they can write files into the shared storage without needing the user to do anything except select the folder once.

[xypwn]

Andy suggested the name NewFolderSave on Discord, which would be a name that is more consistent with the current API. It could give the callback function a listable URI, which can be checked for persistent writability on subsequent uses. This would be very similar to NewFolderOpen.

[Jacalz]

This is a great idea (using the NewFolderSave naming). Thanks for sharing 👍