Added get_line_prefix attribute to Window.

This is a refactoring that opens up many possibilities.
- Line wrapping (soft or hard) can insert or remove whitespace in front of the
  line, or insert some symbols in front. Like the Vim "breakindentopt" option.
- Single line prompts also support line continuations now.
- Line continuations can have a variable width.
- Possibility to implement "breakindent" on top of this.