Allow ExportPlugin to add export messages; use in C# export plugin

[raulsntos]

• Expose EditorExportPlatform::add_message as a method in ExportPlugin so plugins can add messages that will be displayed in the export log at the end of exporting.
• Use the new ExportPlugin::add_message in the C# export plugin to add error messages instead of a custom error dialog. Integrates the .NET error messages with the export log dialog so only one dialog is displayed.

Alternative design

We could alternatively expose the method in EditorExportPlatform and add a get_export_platform method to ExportPlugin so we could retrieve the export platform from the plugin.

[dsnopek]

Alternative design

We could alternatively expose the method in EditorExportPlatform and add a get_export_platform method to ExportPlugin so we could retrieve the export platform from the plugin.

I've been planning to eventually make a PR to add an EditorExportPlatformExtension class that would allow GDExtensions to add new EditorExportPlatform's. (I started this in a branch a few months ago, but haven't found the time to finish it.)

This would be helpful in allowing folks to add support for exporting to console, for example, without needing to make a custom build of the editor.

In any case, as part of that work, we'd need to expose EditorExportPlatform::add_message() anyway. So, I personally quite like the alternative design, otherwise, we'll end up with two ways to do add_message()

[raulsntos]

That's a good point, the alternative design would also supersede #90556 since having access to the EditorExportPlatform would not require us to expose another way to get the OS name from ExportPlugin.

That said, as long as EditorExportPlatform is not accessible from EditorPlugin then even if both of these add_message methods are exposed, only one of them would be available depending on where you are.

[dsnopek]

That said, as long as EditorExportPlatform is not accessible from EditorPlugin then even if both of these add_message methods are exposed, only one of them would be available depending on where you are.

Ah, yeah, that makes sense. Having EditorPlugin::add_message() available makes it more convenient to use when implementing an EditorPlugin, and there isn't really any harm in having it both places. :-)

[raulsntos]

• Superseded by Allow adding custom export platforms using scripts / GDExtension. #90782.

I'll make a PR for the C# changes in this PR after #90782 is merged.