[Feature Request] Align lyrics to middle of font 'x-height' instead of baseline

[Abraham Lee]

Dev Team,

I was wondering if you have any insight concerning the possibility
of/complications with changing the default Y-offset of LyricText to
be aligned on vertical center of the font's x-height. The Dynamics
characters do something similar already (roughly aligned on the vertical
center of Emmentaler's "m" glyph[1]), so why not lyrics? Seems like this
would bring a visual improvement over the current Y-offset of zero (i.e.,
the font's baseline).

The "x-height" is a standard font metric found in the metadata of the text
font, so you normally wouldn't need to calculate it on-the-fly (assuming
the designer set it correctly), but maybe calculating it on-the-fly would
be better, only reverting to the metadata value should the font not contain
the "x" glyph (rare, I'm sure)?

I guess this might only be helpful when you want the lyrics to be
vertically centered between two staves (like in a hymnal), so maybe there's
a better way to do this, like the Dynamics do, keeping its context mid-way
between staves?

Just some thoughts. Here's the relevant doc reference:

http://lilypond.org/doc/v2.19/Documentation/notation/flexible-vertical-spacing-within-systems#within_002dsystem-spacing-properties

Thanks for all you do!
- Abraham

P.S. Also, why is LyricText.font-size = #1 by default? Why not #0? Not
complaining. Just trying to understand.

[1] I say "roughly" because DynamicText doesn't actually query the Y-extent
of the "m" glyph. It has a hard-coded value that is close enough to match
the mid-height of Emmentaler's "m" (see line 857 of scm/define-grobs.scm
and line 972 of scm/output-lib.scm).