IME message generation/handling (WM_IME_*)

To support IME/IMM for Asian language input, we have to implement the standard IME message (WM_IME_*) generation in default handling of window procedure.

For example, the default handling of WM_ACTIVATE generates WM_IME_SETCONTEXT message. The default handling of WM_IME_SETCONTEXT message generates WM_IME_NOTIFY message.

Opening:

WM_ACTIVATE --> WM_IME_SETCONTEXT(wParam = 1; lParam = 0xC000000F) --> WM_IME_NOTIFY(wParam = 2; lParam = 0).

Closing:

WM_IME_SETCONTEXT(wParam = 0; lParam = 0xC000000F) --> WM_IME_NOTIFY(wParam = 1; lParam = 0).