add clear screen support with CTRL-L

[lu-maca]

#229

Changes:

• add to *keyboard.h support for CTRL-L;
• a CTRL-L in Terminal::Keypressed() clears the screen by flushing the escape string on the ostream;
• after a CTRL-L, restore the session by calling session.Prompt()

TODO: add escape string for windows

[daniele77]

Thank you for your contribution, Luca.
I just need you to get rid of the std::string_view since this is a C++14 library, and std::string_view is in the C++17 standard.
Sorry, but we need to keep compatibility with C++14.
Thanks.

[lu-maca]

Hi @daniele77,
no problem for me.
I'm having some troubles with including the change for windows. I'm not sure if there is a way to clear the terminal without invoking some system call. Would it be ok?

[daniele77]

After a quick check, I made Windows ctrl-L work using a Windows API.
Unfortunately, the library currently does not distinguish between Windows and Linux "terminal" (class Terminal is unique, as you can see), so I must put in place some design refractory to the class structure.
Besides, ctrl-L doesn't work on the telnet terminal either. I hope this can be fixed, I'll look at it in the next few days.

[lu-maca]

After a quick check, I made Windows ctrl-L work using a Windows API. Unfortunately, the library currently does not distinguish between Windows and Linux "terminal" (class Terminal is unique, as you can see), so I must put in place some design refractory to the class structure. Besides, ctrl-L doesn't work on the telnet terminal either. I hope this can be fixed, I'll look at it in the next few days.

This was my doubt: I was not sure if using Windows API would broke something on other sides. Linux can have this problem too: escape sequences for clearing the terminal are not - as far as I know - standardized for different terminals

[daniele77]

I've already implemented (not yet pushed on the server, though) all the machinery needed to make ctrl-L work on Linux, Windows, and telnet.
However, I've used the escape sequence for Linux (and telnet), and I don't know if it will work on different terminals.
I'm going to push the feature in the next few days, after the usual tests and documentation update.