LibGfx+Userland: Remove dependency on GUI::TabWidget from StylePainter

Move TabPosition into its own file, and using it into the global
namespace the same way we do for Gfx::Orientation. This unbreaks the gn
build, and out of tree builds.

Userland/Libraries/LibGfx/TabPosition.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2023, Andrew Kaster <akaster@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Gfx {
+
+enum TabPosition {
+    Top,
+    Bottom,
+    Left,
+    Right,
+};
+
+}
+
+using Gfx::TabPosition;