This change adds missing OS.g_list_free() calls in ToolItem with
DROP_DOWN style, after calls to gtk_container_get_children(). This
prevents native memory leaks.

Change-Id: Ibba2eacaab14c93b6c0e03efb0d9ebc728bdb99a
Signed-off-by: Simeon Andreev <simeon.danailov.andreev@gmail.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.swt/+/180522
Reviewed-by: Andrey Loskutov <loskutov@gmx.de>
Tested-by: Platform Bot <platform-bot@eclipse.org>