Multiple Mixer Channel Selection

[regulus79]

This PR adds the ability to select multiple mixer channels at once and do simple operations on all of them, such as sending, muting, deleting, and changing color.

Implementation

• The currently selected mixer channels are stored in a std::set within MixerChannelView
• Lots of methods which used to work on a single channel have been changed to work with the entire selection of channels In particular,
  • Send/unsend
  • Changing color
  • Moving left/right
  • Deleting selected channels
  • Muting/soloing

Known Bugs

• Bulk solo is buggy
• Bulk moving left/right is buggy when some unselected channels have send/receives. I'm not sure why.

Now that I think about it, this code is really messy and this probably isn't the best way to handle it. If you all don't think this is a good PR, I can wait for someone else to do it better lol.