cover: Add placeholder icon

cartridges/ui/cover.blp

@@ -8,21 +8,43 @@ template $Cover: Adw.Bin {
     maximum-size: bind picture.height-request;
     tightening-threshold: bind picture.height-request;
 
-    Adw.Clamp {
+    child: Adw.Clamp {
       unit: px;
       maximum-size: bind picture.width-request;
       tightening-threshold: bind picture.width-request;
 
-      Picture picture {
+      child: Stack {
+        visible-child-name: bind $_get_stack_child(template.paintable) as <string>;
+        overflow: hidden;
+
+        StackPage {
+          name: "cover";
+
+          child: Picture picture {
             paintable: bind template.paintable;
             width-request: 200;
             height-request: 300;
             content-fit: cover;
+          };
+        }
+
+        StackPage {
+          name: "icon";
+
+          child: Image {
+            icon-name: bind template.app-icon-name;
+            pixel-size: 80;
+
+            styles [
+              "dimmed",
+            ]
+          };
+        }
 
         styles [
           "card",
         ]
-      }
+      };
-    }
+    };
   };
 }

cartridges/ui/cover.py

@@ -3,7 +3,7 @@
 
 from gi.repository import Adw, Gdk, GObject, Gtk
 
-from cartridges.config import PREFIX
+from cartridges.config import APP_ID, PREFIX
 
 
 @Gtk.Template.from_resource(f"{PREFIX}/cover.ui")
@@ -13,3 +13,8 @@ class Cover(Adw.Bin):
     __gtype_name__ = __qualname__
 
     paintable = GObject.Property(type=Gdk.Paintable)
+    app_icon_name = GObject.Property(type=str, default=f"{APP_ID}-symbolic")
+
+    @Gtk.Template.Callback()
+    def _get_stack_child(self, _obj, paintable: Gdk.Paintable | None) -> str:
+        return "cover" if paintable else "icon"