Creates pure conversion for Flag::calc_y_offset (issue 5934050)

[k-ohara5a5a]

LGTM,
but if the solution is to avoid looking at the stem extent, then there
is another case to solve with tremolo flags
 << { \aikenHeads f'4:32 }  \\ { \aikenHeads f' } >>
and then beams
 << { \aikenHeads e'8 f' s4 }  \\ { \aikenHeads e'8 f' s4 } >>
...


http://codereview.appspot.com/5934050/diff/1/input/regression/flag-stem-begin-position.ly
File input/regression/flag-stem-begin-position.ly (right):

http://codereview.appspot.com/5934050/diff/1/input/regression/flag-stem-begin-position.ly#newcode8
input/regression/flag-stem-begin-position.ly:8: << { \aikenHeads f'8 }
\\ { \aikenHeads f'8 } >>
We already have a very similar ‘collision-head-solfa-fa.ly’

http://codereview.appspot.com/5934050/