Port away from Quotient::Omittable

src/definitions.h

@@ -0,0 +1,10 @@
+// SPDX-FileCopyrightText: 2024 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#if Quotient_VERSION_MINOR > 8
+#define Omittable std::optional
+#define none Quotient::none
+#else
+#define Omittable Quotient::Omittable
+#define none std::nullopt
+#endif