[SDK][USER32] Implement WM_IME_SYSTEM.IMS_SOFTKBDONOFF (#7950)

Supporting soft keyboard will improve IME
usability.
JIRA issue: CORE-19268
- Define IMS_SOFTKBDONOFF constant as
  0x13 in <immdev.h>.
- Implement
  WM_IME_SYSTEM.IMS_SOFTKBDONOFF
  message handling in ImeWnd_OnImeSystem.

sdk/include/ddk/immdev.h

@@ -103,6 +103,7 @@ typedef struct tagGUIDELINE {
 #define IMS_CONFIGURE           0x0D
 #define IMS_SETOPENSTATUS       0x0F
 #define IMS_FREELAYOUT          0x11
+#define IMS_SOFTKBDONOFF        0x13
 #define IMS_GETCONVSTATUS       0x14
 #define IMS_IMEHELP             0x15
 #define IMS_IMEACTIVATE         0x17

win32ss/user/user32/misc/imm.c

@@ -618,8 +618,25 @@ User32DoImeHelp(PIMEUI pimeui, WPARAM wParam, LPARAM lParam)
     return ret;
 }
 
+static BOOL CALLBACK
+ImeWnd_SwitchSoftKbdProc(_In_ HIMC hIMC, _In_ LPARAM lParam)
+{
+    DWORD dwConversion, dwSentence, dwNewConversion;
+
+    IMM_FN(ImmGetConversionStatus)(hIMC, &dwConversion, &dwSentence);
+
+    if (lParam)
+        dwNewConversion = dwConversion | IME_CMODE_SOFTKBD;
+    else
+        dwNewConversion = dwConversion & ~IME_CMODE_SOFTKBD;
+
+    if (dwNewConversion != dwConversion)
+        IMM_FN(ImmSetConversionStatus)(hIMC, dwNewConversion, dwSentence);
+
+    return TRUE;
+}
+
 /* Handles WM_IME_SYSTEM message of the default IME window. */
-/* Win: ImeSystemHandler */
 static LRESULT ImeWnd_OnImeSystem(PIMEUI pimeui, WPARAM wParam, LPARAM lParam)
 {
     LRESULT ret = 0;
@@ -720,8 +737,8 @@ static LRESULT ImeWnd_OnImeSystem(PIMEUI pimeui, WPARAM wParam, LPARAM lParam)
             ret = IMM_FN(ImmFreeLayout)((HKL)lParam);
             break;
 
-        case 0x13:
-            FIXME("\n");
+        case IMS_SOFTKBDONOFF:
+            IMM_FN(ImmEnumInputContext)(0, ImeWnd_SwitchSoftKbdProc, lParam);
             break;
 
         case IMS_GETCONVSTATUS: