Meson: Flatten directory hierarchy

Keep tests/ and icons/ but put the rest into the toplevel.

The previous setup was so different from the rest of the bunch that
it regularly stuck out in maintenance tasks.

Change-Id: I69821be6268f69682353a388f6bb1fa343215303
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/mesonprojectmanager/toolssettingspage.cpp

@@ -0,0 +1,21 @@
+// Copyright (C) 2020 Alexis Jeandet.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0+ OR GPL-3.0 WITH Qt-GPL-exception-1.0
+
+#include "toolssettingspage.h"
+
+#include "mesonpluginconstants.h"
+#include "toolssettingswidget.h"
+
+namespace MesonProjectManager {
+namespace Internal {
+
+ToolsSettingsPage::ToolsSettingsPage()
+{
+    setId(Constants::SettingsPage::TOOLS_ID);
+    setDisplayName(tr("Tools"));
+    setCategory(Constants::SettingsPage::CATEGORY);
+    setWidgetCreator([]() { return new ToolsSettingsWidget; });
+}
+
+} // namespace Internal
+} // namespace MesonProjectManager