[partition] Add convenience function formatByteSize

We want to use the KPMCore function consistently, but Calamares
uses a qint64 most of the time. Centralize the cast to double
in one place in the code.

src/modules/partition/core/SizeUtils.h

@@ -0,0 +1,27 @@
+/* === This file is part of Calamares - <https://calamares.io> ===
+ *
+ *   SPDX-FileCopyrightText: 2021 Adriaan de Groot <groot@kde.org>
+ *   SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ *   Calamares is Free Software: see the License-Identifier above.
+ *
+ */
+
+#ifndef PARTITION_CORE_SIZEUTILS_H
+#define PARTITION_CORE_SIZEUTILS_H
+
+#include <kpmcore/util/capacity.h>
+
+/** @brief Helper function for printing sizes consistently.
+ *
+ * Most of Calamares uses a qint64 for partition sizes, so use that
+ * parameter type. However, the human-visible formatting doesn't need
+ * to bother with one-byte accuracy (and anyway, a double has at least 50 bits
+ * at which point we're printing giga (or gibi) bytes).
+ */
+static inline QString formatByteSize( qint64 sizeValue )
+{
+    return Capacity::formatByteSize( static_cast< double >( sizeValue ) );
+}
+
+#endif  // PARTITION_CORE_SIZEUTILS_H