Support explicit inheritance of extracts

[bwl21]

Problem

As of now all Extracts inherit from extract 0. But this might not be sufficient, since there might be extract groups with common parts. E.G.

• particular voice for two different instruments
• vertical extract

in the following examlple. 4,5,6 may be like 1,2,3 but only have countnotes ...

0   -> 1  -> 4
     -> 2  -> 5
     -> 3  -> 6

Solution approach

• [ ] have a parameter extract.x.parent which is a list of extracts.

• [ ] do not support transitive inheritance

• [ ] inherit from left to right, the last parameter wins

• [ ] Implement this as an extra method which can be used in harpnotes.rb line 1667 as well as in get_config_values in controller_command_definitions:690