Add support for custom dash definition

As mentioned in 304382 it can be really useful to have custom dash definitions for lines. You can use them to avoid collisions, like in bar 21 of the attached Lilypond engraving of Marc’s MuseScore 2 demo (https://musescore.com/marcsabatella/reunion), but you can also use them for editorial tricks like if a composer draws incomplete slurs and ties, you can use this to signify the parts you added.

Lilypond does this like this: A dash definition is a list of 4-tuplets (start stop dash-length length) for the Beziér parameter, so with the parameter t in (start stop) we get dashes of length dash-length with a separation of length – dash-length.

This might not be a very important feature, but it would be nice to have it.