"Variant" Beat/Bassline Editor Proposal

This proposal is to allow the Beat/Bassline Editor to allow variants within the same BB Editor track.

**Current Behavior**

> Main Beat (example)
> 
>
> Main Beat2 (example)
> 

In the current behavior of the Song Editor, the only way to have a variation of a beat is to create a separate Beat/Bassline Track.

> Original (Main Beat) and variation (Main Beat2) taking up two tracks
> 

The Song Editor already has a Copy/Paste option for BB Editor blocks:

> Copy/Paste a BB Editor Pattern
> 

**Proposal**
The proposal is to make the Copy/Paste option required for cloning a particular BB Editor pattern, but clicking on a new "blank" area will create an BB Editor with the same instrument tracks, but without the patterns filled in. (similar to how a new or copied Piano Roll pattern behaves)

> Clicking a "blank" area
> 

This proposal is to allow the BB Editor to behave like the Piano Roll in the sense that the patterns internal to that particular BB are stored unique to the Song Editor block:

> Two different beats on one Song Editor Track
> 

This would work in conjunction with Siloed BB Editor proposal #453 as the available instruments would be bound to that particular Song Editor track.

Start by reading related proposal #453 and tracing the existing Copy/Paste behavior between the Song Editor and Beat/Bassline Editor. The issue names the blank-area creation and pattern-cloning behaviors to compare with Piano Roll patterns; done means variants can share one Song Editor track while retaining independent patterns and instrument availability.

Written by the indexing model from the issue text.