[NTUSER][IMM32] Use Imm32CurrentPti and pool (#4356)

- Use newly-defined Imm32CurrentPti() instead of NtCurrentTeb()->Win32ThreadInfo.
- THREADSTATE_GETTHREADINFO is same as THREADSTATE_UNKNOWN18.
- Use the paged pool to allocate HIMC rather than heap.
- Fix and improve ImmDestroyContext function.
CORE-11700

win32ss/include/ntuser.h

@@ -1215,7 +1215,7 @@ typedef struct tagIMEUI
 /* Window Extra data container. */
 typedef struct _IMEWND
 {
-    WND;
+    WND wnd;
     PIMEUI pimeui;
 } IMEWND, *PIMEWND;
 
@@ -2570,8 +2570,7 @@ enum ThreadStateRoutines
     THREADSTATE_ISWINLOGON,
     THREADSTATE_ISWINLOGON2,
     THREADSTATE_UNKNOWN17,
-    THREADSTATE_UNKNOWN18,
-    THREADSTATE_GETTHREADINFO = 100, /* FIXME: Delete this HACK */
+    THREADSTATE_GETTHREADINFO,
     THREADSTATE_PROGMANWINDOW, /* FIXME: Delete this HACK */
     THREADSTATE_TASKMANWINDOW, /* FIXME: Delete this HACK */
 };