Enable building of extensions:

[lukas-h]

enum ExtensionType{
    popup,
    sidebar,
    // …
}

class PageExtension {
    PageExtension({
        this.activeOnPages,
        this.activeOnJournals,
        this.extensionType,
    });
    Widget buildControlButton(BuildContext context, PageState state, bool opened);

    Widget buildBody(BuildContext context, PageState state); // only builds when opened
}