Context aliases

[Erik Sandberg]

hi,

In my quest for softcoding context-defs, I will need to revisit the \alias 
system. This is not on the top of my todo, but I'd like to hear any protests 
early.

I suggest that the \alias mechanism is scrapped altogether, and replaced with 
something else:
- junk Bottom alias, instead make is_bottom_context accessible from scheme.
- other aliases are replaced with appropriate boolean context properties, with 
more intuitive names (e.g. property 'isLyricMelody replaces current \alias 
Voice).

I think this will be an improvement, because we remove the concept of aliases, 
which confuses users (at least it confuses me a bit); in addition we don't 
need to do anything extra to softcode aliases.

All this is post-2.10 of course.

-- 
Erik