Use builtin Clear function for screen clearing

[joelim-work]

Fixes #845

Credit to @horriblename for finding this.

[gokcehan]

@joelim-work @horriblename Thanks for coming up with this patch. It seems that the change from Clear to manual filling was introduced with the change from termbox to tcell in #439 . In the discussion it says Clear has an extra flush so it was changed to manual filling to avoid the flush. I'm not sure if it is still the case with recent versions of tcell. There is also mention of a flicker issue on Windows so this patch needs to be tested on Windows as well. All in all, we need to make sure this patch does not change the behavior of the program in any way.

[joelim-work]

@gokcehan Thanks for mentioning this, I didn't realise there that the behaviour would be different on Windows. I did a test on Windows and I can confirm that the slowness is still an issue.

It looks like using Clear/Fill is semantically different to just using SetContent, it sets s.clear = true which eventually ends up flushing the screen by calling clearScreen: https://github.com/gdamore/tcell/blob/c951371181bb4b1387d9010b8081590ae6a48152/console_win.go#L1159-L1170

I don't think there's any good way around this so I'm going to just close this pull request. Maybe it might be worth adding a comment to explain the Windows issue, but I'll leave that up to you to decide.

[gokcehan]

@joelim-work I have now added a comment, thanks.