[partition] Refactor filling combobox with moint points

- Move to one place which handles the standard mount points
 - While here, introduce explicit "(no mount point)" string
   into the combo box. This is prep-work for issue #951.

src/modules/partition/gui/MountPoints.cpp

@@ -0,0 +1,48 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   Copyright 2014, Aurélien Gâteau <agateau@kde.org>
+ *   Copyright 2016, Teo Mrnjavac <teo@kde.org>
+ *   Copyright 2018, 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/>.
+ */
+
+#include "MountPoints.h"
+
+#include "core/PartUtils.h"
+
+#include "GlobalStorage.h"
+#include "JobQueue.h"
+
+#include <QComboBox>
+
+QStringList
+standardMountPoints()
+{
+    QStringList mountPoints{ "/", "/boot", "/home", "/opt", "/usr", "/var" };
+    if ( PartUtils::isEfiSystem() )
+        mountPoints << Calamares::JobQueue::instance()->globalStorage()->value( "efiSystemPartition" ).toString();
+    mountPoints.removeDuplicates();
+    mountPoints.sort();
+    return mountPoints;
+}
+
+void
+standardMountPoints(QComboBox& combo)
+{
+    combo.clear();
+    combo.addItem( combo.tr( "(no mount point)" ) );
+    combo.addItems( standardMountPoints() );
+}
+