Eliminates side poisition calculations for system-start-text grobs. (issue 7574048)

[janek . lilypond]

Whoah, Mike!  I've understood your patch in first reading! Nice commit
message.

> The spacing of system-star-text-interface implementing grobs used side

typo (system-start-text)

> Also, we change the name of the list of elements used for alignment,
> as elements should only be used for axis groups.

maybe enclose "elements" (from the second line) in quotes?  From what i
understand, this sentence says that 'elements' is a special identifier
that should only be used for axis groups.

anyway, LGTM :)

thanks!
Janek

https://codereview.appspot.com/7574048/