#5656 Warn about accessing the Global context explicitly

[Dan Eble]

1: Add regression test for \new Global
This passes without changes: LilyPond issues a warning about \new Global.

2: Warn about trying to access the Global context explicitly
Warn for \context Global and \set Global.property as for \new Global.

https://codereview.appspot.com/567050043

[Anonymous]

Passes make, make check and a full make doc.

[Anonymous]

Patch on countdown for Jan 14th

[Anonymous]

This has reached Push status but I am not sure if this really has been approved or not, if it has not can you set this back to Countdown (otherwise please push)

[Dan Eble]

commit aec2817edb2f343e971ddba5dc69ce8007fcef3e (HEAD -> staging, origin/staging)
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Wed Jan 8 17:27:39 2020 -0500

    Issue 5656/2: Warn about trying to access the Global context explicitly

    Warn for \context Global and \set Global.property as for \new Global.

commit 01d50bf64e926c80c51dcbb91a63d2d151ac7a5f
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Tue Jan 7 12:18:08 2020 -0500

    Issue 5656/1: Add regression test for \new Global