Fix button rendering on EditorProperty with bottom editors

[pocchi the dev]

Exports that are internally set via EditorProperty.set_bottom_editor, are not rendered properly. Examples include exports set via @export_multiline and flag exports. This is made difficult due to the fact that EditorProperty does not expose bottom_editor to GDScript, so it is difficult to detect when an EditorProperty requires a different way to render buttons.

To fix this issue, we need to detect if an EditorProperty's child is set via set_bottom_editor, and set it again in the renderer when the button is injected.