Enhancement: always print key signature grobs, even for keys with no sharps or flats

[Paul Morris]

Greetings bug squad,

Say you want to override all key signature grobs, either to use a non-standard 
key signature system, or to enhance the traditional key signature system, as in 
this snippet:  
http://lsr.dsi.unimi.it/LSR/Item?id=856

...unfortunately, for keys with no sharps or flats like C major and A minor, 
LilyPond does not always give you a key signature grob to override.

For these keys LilyPond *does* put an empty (invisible) key signature grob at 
the beginning of the first line of music, and also any time the key changes to 
a key with no sharps or flats.  However, LilyPond does *not* include an empty 
key signature grob at the beginning of any lines of music after the first.

The attached files illustrate this.  On the lower staff a simple custom key 
signature engraver replaces all key signature grobs with a solid circle.  (Just 
for demonstration, not an actual use case, obviously.)  Note how there is no 
black circle at the beginning of the 2nd and 4th lines of music (i.e. at 
measures 13 and 39).

Of course, this is not a problem for standard use, so this is an enhancement 
request rather than a bug, and no doubt a very low priority one at that.  
However, to better support overriding key signatures, I think it would be an 
improvement if LilyPond were more consistent and always provided a key 
signature grob to override, regardless of the key.

(This did come up on the user list in relation to the snippet linked above:
http://lists.gnu.org/archive/html/lilypond-user/2012-11/msg00545.html )

Thanks for considering,
-Paul

custom-key-sig-demo.ly
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