[packagechooser] Initial support for appstream items

- Use *appstream* as key in one of the items for the package-
   chooser to load data from the AppStream cache in the system.
 - Usable for some applications; for DE-selection not so much.
 - Currently unimplemented.

src/modules/packagechooser/PackageModel.cpp

@@ -98,6 +98,15 @@ PackageItem::fromAppData( const QVariantMap& item_map )
 }
 #endif
 
+#ifndef HAVE_APPSTREAM
+PackageItem
+PackageItem::fromAppStream( const QVariantMap& item_map )
+{
+    cWarning() << "Loading AppStream data is not supported.";
+    return PackageItem();
+}
+#endif
+
 
 PackageListModel::PackageListModel( QObject* parent )
     : QAbstractListModel( parent )