Support Title Stacking (CSI 22 t / CSI 23 t)

Now some applications set terminal window title using OSC sequence.
For example, Vim (with "set title" option) sets the message string "Thanks for flying Vim" to the window title when it shuts down.
But some people consider it as "bad manner", so they try to fix this behavior using title reporting (CSI 20 t / CSI 21 t), but this feature has security risk.

I Think this problem can be solved if Title Stacking feature will be widly supported.

Here is example settings for Vim.

.vimrc
```
set title
let &t_ti = &t_ti . "\e[22;0t"
let &t_te = "\e[23;0t" . &t_te
```

The next release of Tera Term will support Title Stacking.
http://sourceforge.jp/projects/ttssh2/scm/svn/commits/5096