Initial Prestel support.

Basic Prestel support based on the "Prestel Terminal Specification"
Issue 2 by the Post Office.

Known issues:
Modifying an existing line does not update the row after it.
Double height is not supported.
Keyboard mapping kinda sucks (looking at you #/Return).
Concealed is always shown.
Remote programming sequences aren't supported.

There appears to be something wrong with the End of the Line title
and menu screens, but maybe nelgin did them wrong.