[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/ItemAppStream.cpp

@@ -0,0 +1,33 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   Copyright 2019, Adriaan de Groot <groot@kde.org>
+ *
+ *   Calamares is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation, either version 3 of the License, or
+ *   (at your option) any later version.
+ *
+ *   Calamares is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU General Public License for more details.
+ *
+ *   You should have received a copy of the GNU General Public License
+ *   along with Calamares. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @brief Loading items from AppData XML files.
+ *
+ * Only used if QtXML is found, implements PackageItem::fromAppData().
+ */
+#include "PackageModel.h"
+
+#include "utils/Logger.h"
+#include "utils/Variant.h"
+
+PackageItem
+PackageItem::fromAppStream( const QVariantMap& map )
+{
+    cWarning() << "AppStream loading not implemented.";
+    return PackageItem();
+}