[libcalamares] Start refactor of running-a-process support

The background idea is that, while CalamaresUtils::System::runCommand()
is a useful general API, it is
- still missing flexibility
- lacking a way to process output from the command "as it happens"

Waiting until the process ends, and then reading all stdout, is
inconvenient for processes that produce a **lot** of output,
and also makes it impossible to report progress. One module
in calamares-extensions has its own run-a-process implementation
for reading output, and this branch aims to introduce something
similar into Calamares core.

src/libcalamares/utils/Runner.cpp

@@ -0,0 +1,38 @@
+/* === 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.
+ *
+ *
+ */
+
+#include "Runner.h"
+
+namespace
+{
+}
+
+namespace Calamares
+{
+namespace Utils
+{
+
+struct Runner::Private
+{
+    QStringList m_command;
+};
+
+Runner::Runner() {}
+Runner::Runner( const QStringList& command )
+    : d( std::make_unique< Private >() )
+{
+    d->m_command = command;
+}
+
+Runner::~Runner() {}
+
+
+}  // namespace Utils
+}  // namespace Calamares

src/libcalamares/utils/Runner.h

@@ -0,0 +1,54 @@
+/* === 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 UTILS_RUNNER_H
+#define UTILS_RUNNER_H
+
+#include "CalamaresUtilsSystem.h"
+
+#include <QObject>
+#include <QStringList>
+
+#include <chrono>
+#include <memory>
+#include <optional>
+
+namespace Calamares
+{
+namespace Utils
+{
+
+using RunLocation = CalamaresUtils::System::RunLocation;
+
+class Runner : public QObject
+{
+    Q_OBJECT
+
+public:
+    Runner();
+    Runner( const QStringList& command );
+    virtual ~Runner() override;
+
+    Runner& setCommand( const QStringList& command );
+    Runner& setLocation( RunLocation r );
+    Runner& setWorkingDirectory( const QString& directory );
+    // TODO: maybe with QDir as well?
+    Runner& setTimeout( std::chrono::seconds timeout );
+    Runner& setInput( const QString& stdin );
+
+private:
+    struct Private;
+    std::unique_ptr< Private > d;
+};
+
+}  // namespace Utils
+}  // namespace Calamares
+
+#endif