Applying tweaks to one voice in \partCombine

When using \partCombine, commands applying to a Voice context affect both parts since when are merged in the same Voice. This snippet defines a music function to apply tweaks to all elements of a certain kind in a music expression. This can be used to apply modifications to all grobs caused by elements of a music expression regardless of the context structure. Possible values for the event-class argument are listed at the Internals Reference