Add getUserVisibleLines() for acquring line number of top and bottom lin...

Not sure if you like it, but it will definitely come in handy in some use cases, e.g.:
* `H` `M` `L` key bindings in Vim, `Alt-0, Alt-r``Alt--, Alt-r` in Emacs
*  Emacs-like visual bell
*  Extracting text in current user visible window

You can see how it behaves in the attached demo, just let me know what you think.