Prepare xdnd.c to be in PE file and use ntdll interface for Unicode conversion.
-- v2: winex11: Directly use ntdll for utf8 conversion. winex11: Move Unicode conversion out of string_from_unicode_text. winex11: Move Unicode conversion out of unicode_text_from_string. winex11: Use inline intersect_rect helper instead of IntersectRect. winex11: Move XdndLeave event handler to event.c. winex11: Move XdndDrop event handler to event.c. winex11: Move XdndPosition event handler to event.c. winex11: Move XdndEnter event handler to event.c.