refactor and clean up app library

examples/app_library/app_grid/imp.rs

@@ -0,0 +1,26 @@
+use gtk4::subclass::prelude::*;
+use gtk4::{gio, glib, GridView};
+use once_cell::sync::OnceCell;
+
+#[derive(Default)]
+pub struct AppGrid {
+    pub app_grid_view: OnceCell<GridView>,
+    pub app_model: OnceCell<gio::ListStore>,
+    pub app_sort_model: OnceCell<gtk4::SortListModel>,
+    pub search_filter_model: OnceCell<gtk4::FilterListModel>,
+    pub group_filter_model: OnceCell<gtk4::FilterListModel>,
+}
+
+#[glib::object_subclass]
+impl ObjectSubclass for AppGrid {
+    // `NAME` needs to match `class` attribute of template
+    const NAME: &'static str = "AppGrid";
+    type Type = super::AppGrid;
+    type ParentType = gtk4::Box;
+}
+
+impl ObjectImpl for AppGrid {}
+
+impl WidgetImpl for AppGrid {}
+
+impl BoxImpl for AppGrid {}