FootnoteItem positioning

[Trevor Daniels]

While considering how to add a footnotes to TextScript I came across
this, which could be intended or a bug.  If the positioning number pair
is set to #'(0 . 0) the footnote mark is located in both x and y directions
on the centre of the TextScript object, but if the values are set to any
non-zero value the offsets are calculated from the top-right corner of the
TextScript object.  So to position the mark where it would normally be
after text you have to set the values to any tiny non-zero value.

Here's an example to play with:

\book {
  \header { tagline = ##f }
  \relative c' {
    a'4 b8
    \once \override Staff.FootnoteItem #'annotation-line = ##f
    \footnote  #'(0.0001 . 0.0001) #'TextScript \markup { "Prout has rall." } 
\default
    e^\markup { "rit." } c4 d4
  }
}

It would be better if #'(0 . 0) resulted in the correct positioning for 
TextScript.

Trevor