Qml/Main: show placeholder if there's no mods

src/leek/qml/Main.qml

@@ -30,6 +30,14 @@ Kirigami.ApplicationWindow {
 
             delegate: ModCardDelegate {}
             model: QModListModel
+
+            Kirigami.PlaceholderMessage {
+                anchors.centerIn: parent
+
+                visible: modsView.count === 0
+                text: "There's no mods installed"
+                explanation: "Install a mod, and it will show up here"
+            }
         }
     }