Use the correct mouse cursor theme on GNOME

Relies on a working desktop settings portal (xdg-desktop-portal-gtk)

glfw/linux_desktop_settings.h

@@ -0,0 +1,14 @@
+/*
+ * Copyright (C) 2021 Kovid Goyal <kovid at kovidgoyal.net>
+ *
+ * Distributed under terms of the GPL3 license.
+ */
+
+#pragma once
+
+#include "dbus_glfw.h"
+#include "internal.h"
+
+
+void glfw_initialize_desktop_settings(void);
+void glfw_current_cursor_theme(const char **theme, int *size);