Allow trill placement inside of the staff

[craigsapp]

Trills can be placed above or below the staff but not on the staff ("within" in MEI terminology). Allow for trill marks to be placed within the staff:

**kern
*clefG2
=
*^
1ggT 1ccc	4f
.	4g
.	4e
.	4d
*v	*v
==
*-

Renders as:

The trill is for the 1gg note, so placing it above instead implies that the 1ccc is trilled. The trill mark must go on the staff in such cases.

Probably controlled with a layout command, such as:

LO:TR:c

where c would mean place the trill in the "center" of the staff.