New bug priority: postponed

[Graham Percival]

About six months ago, there was discussion about a priority level below 
"low"; at the time I argued against it.  I've now changed my mind and 
added a "Priority-Postponed" tag.

Here's my interpretation of the bug priorities:

High, Regression: fix within one or two releases
Medium: fix before next big
Low: might be fixed in one or two stable releases
Postponed: fix would be difficult without major restructuring of the 
lilypond internals, and/or there is no developer actively working in 
this area.  IMO, all ancient notation bugs qualify for this, as do 
esoteric collision bugs.


Cheers,
- Graham