Horizontal scroll window for long lines

[MirrorPusher]

Need the possibility to deal with lines > 80 characters, e.g. TI Basic source code listing or content clipped from host keyboard.

Solution:

• Refactor code for dealing with long lines.
• Introduce "vwco" = View Window Column Offset
• Implement horizontal scroll "F5" that shows frame buffer content starting column X where X is increased from 0 to 250
• Ruler should show sync with vwco, so that it for example starts with indicator '6' for column 60
• Line/Column indicator should deal with 3 digits length for column and line length