[WIN32K][USER32] Change mechanism for storing dialog info pointer

DLGWINDOWEXTRA is 30 (both on win32 and win64). This has storage for the following entries: DWLP_MSGRESULT (0), DWLP_DLGPROC (8), DWLP_USER (16)
We used to store the dialog info pointer using SetWindowLongPtr (DWLP_ROS_DIALOGINFO == DWLP_USER+sizeof(ULONG_PTR) == 24), which was fine on win32, but failed on win64, since there wasn't enough space left (24 + 8 = 32 > 30).
Rewrite the way the DLGINFO pointer is stored, by adding an additional field to the WND structure and set it using NtUserxSetDialogPointer (which is what it is for).
Also fix too small cbWndExtra for the button class.

win32ss/include/ntuser.h

@@ -726,6 +726,8 @@ typedef struct _WND
     PSBINFOEX pSBInfoex; // convert to PSBINFO
     /* Entry in the list of thread windows. */
     LIST_ENTRY ThreadListEntry;
+
+    PVOID DialogPointer;
 } WND, *PWND;
 
 #define PWND_BOTTOM ((PWND)1)