Rewrite using Relm4

Yes, this is a monster commit but at this stage I'm the only one working
on this project anyway. Further commits will follow Best Practises™
again.

src/ui/components/app.rs

@@ -0,0 +1,71 @@
+use gtk4::prelude::GtkWindowExt;
+use libadwaita::ApplicationWindow;
+use libadwaita::prelude::AdwApplicationWindowExt;
+use relm4::{
+  Component, ComponentController, ComponentParts, ComponentSender, Controller, component,
+};
+
+use crate::persist::data_manager::{DataManager, DataManagerError};
+use crate::ui::components::media_type_switcher::MediaTypeSwitcher;
+
+
+
+pub struct App {
+  media_type_switcher: Option<Controller<MediaTypeSwitcher>>,
+}
+
+#[derive(Debug)]
+pub enum AppCmdOutput {
+  DataManagerReady,
+  DataManagerFailed(DataManagerError),
+}
+
+#[component(pub)]
+impl Component for App {
+  type Input = ();
+  type Output = ();
+  type Init = ();
+  type CommandOutput = AppCmdOutput;
+
+  view! {
+    ApplicationWindow {
+      set_title: Some("Zoödex"),
+
+      #[watch]
+      set_content: model.media_type_switcher.as_ref().map(Controller::widget),
+    }
+  }
+
+  fn init(_init: (), root: ApplicationWindow, sender: ComponentSender<App>) -> ComponentParts<App> {
+    sender.oneshot_command(async {
+      match DataManager::init().await {
+        Ok(_) => AppCmdOutput::DataManagerReady,
+        Err(error) => AppCmdOutput::DataManagerFailed(error),
+      }
+    });
+
+    let model = App {
+      media_type_switcher: None,
+    };
+    let widgets = view_output!();
+    ComponentParts { model, widgets }
+  }
+
+  fn update_cmd(
+    &mut self,
+    message: AppCmdOutput,
+    _sender: ComponentSender<App>,
+    _root: &ApplicationWindow,
+  ) {
+    match message {
+      AppCmdOutput::DataManagerReady => {
+        // Only launch child component after data manager is ready.
+        self.media_type_switcher = Some(MediaTypeSwitcher::builder().launch(()).detach());
+      }
+      AppCmdOutput::DataManagerFailed(error) => {
+        // TODO: Show error in GUI.
+        println!("Error occurred in data manager initialization: {:?}", error);
+      }
+    }
+  }
+}