Based on the wine-staging patch winex11-CandidateWindowPos from Felix Yan with works from Muneyuki Noguchi and Sebastian Lackner.
-- v2: win32u: Set host IME composition window position in NtUserShowCaret(). win32u: Set host IME composition window position in set_caret_pos(). win32u: Support setting host IME composition window position for ImmSetCompositionWindow().