This class's window procedure is WinDefImeWindowProc itself,
which handles the following messages.
WM_IMEREQUEST
Default IME Window just sends this message to the appropriate default part
window. If the default part window does not exist, creates that window before
sending the message.
WM_QUERYIMEWINDOWCAPS
Default IME Window procedure sends this message to the default conversion
part window to query the capability of conversion window.
WM_DESTROY
The default IME Window destroys the default part windows which are created by the default IME Window.