Allowing linking between non-genomic visual channels

[sehilyi]

Grammar-wise, we can use linkingId for channels:

tracks: [
   { ..., size: {..., linkingId: 'shared_size' } }, // Track 1
   { ..., size: {..., linkingId: 'shared_size' } }  // Track 2
]

Implementation-wise, we need to allow multiple HiGlass plugin tracks to communicate with each other to send/receive scales, just like how we do with genomic axes.