applets/dict: Add `loadingPlaceHolder` and `errorPlaceHolder`

Show loading status and error message in the config window.

applets/dict/package/contents/ui/ConfigDictionaries.qml

@@ -6,7 +6,7 @@
 
 import QtQuick 2.7
 import QtQuick.Controls 2.5
-import org.kde.kirigami 2.5 as Kirigami
+import org.kde.kirigami 2.19 as Kirigami
 import org.kde.plasma.private.dict 1.0
 
 Page {
@@ -51,4 +51,34 @@ Page {
             }
         }
     }
+
+    Loader {
+        active: dictionariesModel.loading || dictionariesModel.count === 0
+        asynchronous: true
+
+        anchors.centerIn: parent
+        visible: active
+
+        sourceComponent: dictionariesModel.loading ? loadingPlaceHolder : errorPlaceHolder
+
+        Component {
+            id: loadingPlaceHolder
+
+            Kirigami.LoadingPlaceholder {
+                anchors.centerIn: parent
+            }
+        }
+
+        Component {
+            id: errorPlaceHolder
+
+            Kirigami.PlaceholderMessage {
+                anchors.centerIn: parent
+                width: root.width - (Kirigami.Units.largeSpacing * 4)
+                icon.name: "network-disconnect"
+                text: i18n("Unable to load dictionary list")
+                explanation: i18nc("%2 human-readable error string", "Error code: %1 (%2)", dictionariesModel.errorCode, dictionariesModel.errorString)
+            }
+        }
+    }
 }