Improve handling of changing meter

[bwl21]

Request

we need support for piece which changing measues:

1. expose all measures in the legend
2. add notebound annotations for measure changes

solution approach

in moinejf/abc2svg#34 @moinejf explained the flexibility of abc2svg with respect to M:headers.

Therefore, we have the following options:

all generic solution

1. expose a unique liste of M:headers collected throughout the abc file
2. expose a measure change as notebound annotation in the harpnotes

simple solution

As Zupfnoter exposes the content of M:header to the legend we have the following approach:

1. user shall add all Meters to the initial meter, followed by a specification of the initial measure length

   M:3/4 4/4 =4/4

   Zupfnoter needs to strip =.* before rendering the content of M to the harpnotes

2. User shall add meter changes as notebound annotations if needed