[netinstall] Implement a special "blank" item

- If the name of an item is empty, treat it as a
  "separator line", do not paint it like a regular
  item. The branches of the tree just pass it by.

src/modules/netinstall/CMakeLists.txt

@@ -8,6 +8,7 @@ calamares_add_plugin( netinstall
     EXPORT_MACRO PLUGINDLLEXPORT_PRO
     SOURCES
         Config.cpp
+        groupstreeview.cpp
         LoaderQueue.cpp
         NetInstallViewStep.cpp
         NetInstallPage.cpp

src/modules/netinstall/groupstreeview.cpp

@@ -0,0 +1,30 @@
+/* === This file is part of Calamares - <https://calamares.io> ===
+ *
+ *   SPDX-FileCopyrightText: 2022 Adriaan de Groot <groot@kde.org>
+ *   SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   Calamares is Free Software: see the License-Identifier above.
+ *
+ */
+#include "groupstreeview.h"
+
+#include "utils/Logger.h"
+
+#include <QPainter>
+
+void GroupsTreeView::drawBranches(QPainter* painter, const QRect& rect, const QModelIndex& index) const
+{
+    QTreeView::drawBranches(painter, rect, index);
+
+    // Empty names are handled specially: don't draw them as items,
+    // so the "branch" seems to just pass them by.
+    const QString s = index.data().toString();
+    if (s.isEmpty())
+    {
+        QStyleOptionViewItem opt = viewOptions();
+        opt.state = QStyle::State_Sibling;
+        opt.rect = QRect (!isRightToLeft() ? rect.left() : rect.right() + 1, rect.top(), indentation(), rect.height());
+        painter->eraseRect(opt.rect);
+        style()->drawPrimitive(QStyle::PE_IndicatorBranch, &opt, painter, this);
+    }
+}

src/modules/netinstall/groupstreeview.h

@@ -8,4 +8,11 @@
  */
 #include <QTreeView>
 
-using GroupsTreeView = QTreeView;
+class GroupsTreeView : public QTreeView
+{
+public:
+    using QTreeView::QTreeView;
+
+protected:
+    virtual void drawBranches(QPainter*painter, const QRect& rect, const QModelIndex& index) const override;
+};