Refactor: Allow MSA mouseover in mixed layouts

[josiahseaman]

Currently, TileLayout has a set of four containers that all need to be synchronized because they contain different pieces of the same puzzle.

Refactor: Put all synchronized pieces into a single DataSource Object so that mouseover, chunks, download links, layout, and contig spacing can all be coordinated.

1001 Genomes: This will allow me to use MSA code in a Pangenome coordinate frame without requiring separate sequence files for each row. That's important for scaling to 1,000+ genomes.