[USER32] Implement ImeWnd_OnImeSetContext (#4329)

- Modify NtUserSetImeOwnerWindow prototype.
- Add User32GetTopLevelWindow function.
- Add ImeWnd_OnImeSetContext function.
- Implement WM_IME_SETCONTEXT message handling of the IME window.
CORE-11700

sdk/include/ddk/immdev.h

@@ -24,6 +24,12 @@ extern "C" {
 #define IMC_GETSOFTKBDPOS               0x0013
 #define IMC_SETSOFTKBDPOS               0x0014
 
+#define IMMGWL_IMC       0
+#define IMMGWL_PRIVATE   (sizeof(LONG))
+
+#define IMMGWLP_IMC      0
+#define IMMGWLP_PRIVATE  (sizeof(LONG_PTR))
+
 typedef struct _tagINPUTCONTEXT {
     HWND                hWnd;
     BOOL                fOpen;