Treatment of the main window as an object named #mainwin

By default any Liberty BASIC program has a main window which can be hidden using a nomainwin command in the program's code.  This main window is where PRINT statement output appears and where the user responds to INPUT statements, etc.

What's new is that a new default variable named #mainwin and there are some new commands that can be used with it including:

    content$()       - Return the entire contents of the main window as a string.
    cls()            - Clear the main window.
    lines()          - Return the number of lines in the main window.
    line$(n)         - Return the text of line numbered n as a string.
    selection$()     - Return the selected text as a string.
    color(name$)     - Set the text color to be the name of color in name$.
    backcolor(name$) - Set the background color to be the name of color in name$.
    setfocus()       - Raise the main window and make it the active window.
    debug$()         - Return the string "MAINWINDOW".