Inconsistent handling of line editor number in base 0 vs base 1

[FilipVanVooren]

Some routines in Stevie use base 0 for addressing the line of interest. While others use base 1 to address the same.
This inconsistency leads to subtle and very hard to find bugs.

Refactoring is necessary with the target of reaching the following:

• All internal line addressing shall use base 0
• Only when displaying or doing input of a line number on screen, base 1 shall be used
• In all appropriate code places, conversion from base 1 to base 0 is required.