- Added handling of WM_MOUSEACTIVATE message
- Added in_focus field to internal listbox's structure. This flag is set on
  receiving WM_SETFOCUS message and is reseted on WM_KILLFOCUS. All calls to
  GetFocus function were replaced with checks of this flag.
- In LISTBOX_HandleLButtonDown: focus is set only if 'in_focus' flag is
  clear