[bug?] Unnecessary avoid-slur warning

[James Harkins]

It appears that LilyPond 2.18.2 prints a warning if a trill pitch occurs 
under a slur, regardless of the chance of collision.

\relative c' {
 % OK
 \pitchedTrill d2\startTrillSpan ~ e d4. r8\stopTrillSpan
 
 % warning: Ignoring grob for slur: TrillPitchAccidental.  avoid-slur not 
set?
 % But the slur is nowhere near colliding with the trill pitch accidental!
 c4 ( \pitchedTrill d4\startTrillSpan e ~ d4. ) r8\stopTrillSpan
}

This fixes it:

 \override TrillPitchAccidental.avoid-slur = #'inside

But shouldn't this be a default? When would you seriously want a trill 
pitch accidental to be outside the slur? Not to mention that it would make 
no sense to put the trill pitch accidental outside, and the trill pitch 
head inside.

Small issue, work-around-able, but seems a bit untidy.

hjh