Add root_subfolder to FileDialog

[KoBeWi]

Closes godotengine/godot-proposals#3929
root_subfolder will make the specified sub-folder into a "fake root" of the FileDialog. You won't be able to go to parent directory and the path display changes.

Without root_subfolder:

root_subfolder set to Test:

[timothyqiu]

I can still go up by typing .. in the Path box, or select a parent directory file directly by typing ../filename in the File box 😛

Kind of the same issue as #34194. Although that is about the real root, and this one is about the fake root.

[KoBeWi]

I can still go up by typing .. in the Path box

How? I tested that, you can even see on the GIF it's not possible.

select a parent directory file directly by typing ../filename in the File box 😛

Meh, if you don't know filename, then you can't select it. So blocking folder access is enough.

[timothyqiu]

How? I tested that, you can even see on the GIF it's not possible.

I don't know why, but .. works for me...

Meh, if you don't know filename, then you can't select it. So blocking folder access is enough.

That makes sense for "Open". But the restriction leaks for "Save". The user don't have to know about .., mistyping the filename with a leading / breaks it...

p.s. By "break", I mean when the dialog is "confirmed", the user should be able to assume current_filename not pointing to anything outside root_subfolder.

[akien-mga]

Approved on the principle in PR review meeting, but the issues highlighted by @timothyqiu should likely still be fixed before merging.

[KoBeWi]

Rebased. Still can't reproduce the .. bug. As for files, FileDialog seems to have some general problem with picking files. You can e.g. pick a folder when it expects filename and it even emits file_selected signal. The issue existed even before this PR.

[akien-mga]

Let's merge and track remaining/preexisting issues in new bug reports.

[akien-mga]

Thanks!