Add scriptExceptions property/convenience functions (issue 324740043 by address@hidden)

[david . nalesnik]

Reviewers: ,

Message:
I've noticed that setting some properties with this patch doesn't have
an effect: for example, rotation and staff-padding.

Is the best way to approach this issue through the tweak method linked
to in the description on the issue tracker?

Description:
Add scriptExceptions property/convenience functions

These is currently no convenient way to change default
settings for a specific articulation-type.

 -- This commit adds a new context property, scriptExceptions,
which holds custom user settings to modify the default
scriptDefinitions.  The data structure is modeled after
scriptDefinitions and the public variable
default-script-alist.

The settings in scriptExceptions are simply added to the
head of the property-value alists of the indicated scripts.
New settings will be used by virtue of appearing before the
default settings, and there is no need to include settings
of untouched properties because the defaults are still present.

 -- Add two convenience functions for easily modifying settings
for a single articulation type: \addScriptExceptions and
\removeScriptExceptions

 -- Add two regtests and a Changes entry.

Please review this at https://codereview.appspot.com/324740043/

Affected files (+180, -0 lines):
  M Documentation/changes.tely
  A input/regression/script-exceptions.ly
  A input/regression/script-exceptions-adding-removing.ly
  M lily/script-engraver.cc
  M ly/music-functions-init.ly
  M scm/define-context-properties.scm