(Linux) Allow using xdg-desktop-portal for various features on Linux, such as file choosing

[orowith2os]

Describe the project you are working on

I'm not working on the project, but one who packaged it as a flatpak and would appreciate the feature. https://github.com/virtual-puppet-project/vpuppr

Describe the problem or limitation you are having in your project

Godot currently uses its own file chooser, which does not interact well with how Flatpak's sandboxing works.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

It would allow for a dev to use the xdg-desktop-portal file chooser instead of the Godot file chooser, which would make for better host integration and allow for being used inside of Flatpak properly.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

When a user opens a file chooser in a Godot app, it will open the host's file chooser, rather than Godot's.

If this enhancement will not be used often, can it be worked around with a few lines of script?

It may be used often in apps that would allow for packaging as a flatpak on Linux, or want tighter host integration on Linux.

Is there a reason why this should be core and not an add-on in the asset library?

I am not familiar with Godot, so I can not put a reason here other than it would be nice to have for devs willing to enable it for Linux ports.

[Calinou]

Duplicate of #1123.

[orowith2os]

That issue is not exactly a duplicate, but similar. This one is Linux-specific, #1123 is for macOS and Windows (it looks like)

[RokeJulianLockhart]

#4970 (comment)

@Calinou, the scope in the aforelinked issue is very different. They're not equivalent in their current states.

1. Either its title needs to be modified, or this issue needs to be reopened.

2. Allow use of native system file picker dialog on iOS and web #1123 (comment)

[Calinou]

Native file dialogs are supported on Linux since 4.3: godotengine/godot#89735

Enable the corresponding editor setting, then try changing the main font editor setting. This is what I get on KDE X11: