Auto-resize legend column

[alancleary]

When a GCV is first loaded, the legends are in a column on the right and the micro-synteny viewer is in a column on the left. The columns have equal width, which is silly because the legend typically doesn't need nearly that much space. In version 1 of GCV, we handled this by resizing the columns so that the column on the right was just large enough for the legend content, and the micro-synteny viewer got the rest.

@ekcannon requested that this feature be reimplemented in GCV version 2. Make it so!