Option for solo-isolate

### Enhancement Summary
Sometimes I may want a channel or a track to be kept unmuted when I solo a channel or track, i.e. I don't want _solo_ to affect that particular channel or track. For example, if I want to listen only a particular synth track but along with drums and bass tracks, If I have an option to _solo-isolate_ them, I wouldn't need to unmute these tracks manually as I have enabled "solo-isolate" option for them.

If this option is available, _solo_ would not affect _solo-isolate_-enabled track or channel.

#### Justification
It saves time for a user when desired track or channel is kept unmuted while soloing any track or channel.

#### Mockup

1. If I right-click a _channel_ in the **FX Mixer** window, there would be a checkbox "**Solo-isolate**".
2. If I click  button of a track in **Song Editor** or **Beat+Bassline Editor** window, there would be a checkbox "**Solo-isolate**".

Start by tracing the solo controls in the FX Mixer, Song Editor, and Beat+Bassline Editor, including the channel context menu and track solo button mentioned in the issue. Determine how solo currently affects channels and tracks, then define the UI and behavior needed for a Solo-isolate option and verify that isolated items remain unmuted while other items are soloed.

Written by the indexing model from the issue text.