Replace ExportModule.forChild() with provideExporters()

The ExportModule needs a substantial rewrite. It is currently not used anywhere in the app, but it should be used instead of features modules providing ItemActions directly. This big chunk of work will be broken down into a series of issues.

• Replace the deprecated forChild() method with provideExporters() (analogous to settings or main menu entries)
• Rename Exporter.handles() to something more expressive like canExport()