Add solo and mute flags to Part class

At the moment, solo and mute flags are only present on Channel objects. On a Part level, the the Part uses the first Channel it owns for solo and mute even if it has multiple Channels. The addition of the new sequencer requires a more robust approach.

I propose to add solo and mute flags to Part and changing the logic so that

a Channel is muted if: [the Part is muted] or [the Channel is muted]
a Channel is soloed if: [the Channel is not muted] and [the Part is soloed] and ([none of the Part's Channels are soloed] or [this Channel is soloed])