Give ability to change a track's source

[beefoo]

• change a track's item
• change a track's item's start time
• create a "nudge/step" option to replace the prev/next button on tracks that are confusing

[beefoo]

Going to put this on hold. It will require a significant re-design since the interface is designed for one item per pattern. In order to change this, I will need to re-design the "item details" section as well as how the URL is constructed. Also, the interface is optimized to create a strong connection to the original item to further the goal of providing a clear path into the Library's collections. Having multiple items may weaken this connection. Might need to consider a different interface for this.