Give \hide, \omit, \undo more exposure (issue 14430065)

[tdanielsmusic]

This is fine except for learning/tweaks.itely.
Part of this file is explaining about overrides
and later introduces \omit and \hide.  So earlier
examples should retain the \override commands
(or the text will need to be substantially
rewritten to make sense.)

Apart from that I could see only one or two places
where textual changes would be needed (grateful
thanks to Graham for vetoing all attempts to 'talk
through the code'.  He had exactly this sort of
change in mind.)

Trevor


https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely
File Documentation/learning/tweaks.itely (right):

https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely#newcode505
Documentation/learning/tweaks.itely:505: \once \hide TupletNumber
The text above needs to be amended to conform.

https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely#newcode1068
Documentation/learning/tweaks.itely:1068: \omit BarLine
Text above needs to be amended to conform

https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely#newcode1093
Documentation/learning/tweaks.itely:1093: \omit Staff.BarLine
This is getting harder; the text above is trying to
explain the need for specifying the context in an
\override command.

https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely#newcode1102
Documentation/learning/tweaks.itely:1102: shorthand for it called
@code{\omit}:
This is where the \omit command is introduced.
It should not be used earlier.

https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely#newcode1188
Documentation/learning/tweaks.itely:1188: \hide Staff.TimeSignature
This is introduced as an example of overriding.

https://codereview.appspot.com/14430065/diff/1/Documentation/learning/tweaks.itely#newcode1198
Documentation/learning/tweaks.itely:1198: @code{\hide}:
... and this is where \hide is introduced.

https://codereview.appspot.com/14430065/diff/1/Documentation/notation/ancient.itely
File Documentation/notation/ancient.itely (right):

https://codereview.appspot.com/14430065/diff/1/Documentation/notation/ancient.itely#newcode2708
Documentation/notation/ancient.itely:2708: carry flags, make sure to set
@code{\hide Flag} as well.
In passing, I don't think this is necessary now.

https://codereview.appspot.com/14430065/diff/1/Documentation/notation/changing-defaults.itely
File Documentation/notation/changing-defaults.itely (right):

https://codereview.appspot.com/14430065/diff/1/Documentation/notation/changing-defaults.itely#newcode3196
Documentation/notation/changing-defaults.itely:3196: \omit Score.BarLine
Text in para above needs changing too.

https://codereview.appspot.com/14430065/diff/1/Documentation/notation/changing-defaults.itely#newcode3213
Documentation/notation/changing-defaults.itely:3213: \once \hide
NoteHead
Text in para above needs changing too.

https://codereview.appspot.com/14430065/