[USER32] Implement SoftModalMessageBox() based on the original MessageBoxTimeoutIndirectW().

Additional changes:

- Adapt the message-box dialog procedure to correctly take into account
  the presence or absence of a CANCEL button and take appropriate actions.

- Cache the strings retrieved by MB_GetString() (add FIXME notices too).

Author: HBelusca

win32ss/user/user32/user32.spec

@@ -675,7 +675,7 @@
 666 stdcall ShowStartGlass(long)
 667 stdcall ShowWindow(long long) NtUserShowWindow
 668 stdcall ShowWindowAsync(long long) NtUserShowWindowAsync
-669 stdcall SoftModalMessageBox(long)
+669 stdcall SoftModalMessageBox(ptr)
 670 stdcall SubtractRect(ptr ptr ptr)
 671 stdcall SwapMouseButton(long)
 672 stdcall SwitchDesktop(long) NtUserSwitchDesktop