[request] Provide UI to configure display options

[abought]

Most deployments of LocusZoom rely on the developer to choose the display options up front. This works well for a website that browses a known, pre-selected type of data.

For our reusable "bring your own data" tools, the defaults are not universally appropriate: eg a user may want a different line of significance cutoff for eQTLs vs GWAS results.

We should provide UI to control a few common display options. We do not want infinite buttons; some thought should be given to which options are most useful, and how to provide context-sensitive configuration.

Examples:

• GWAS line of significance: provide a configurable context menu popup to change the threshold (orthogonal line data layer)
  • It should be possible to skip the menu and get normal browser behavior, eg, via a modifier key. The feature should be intuitive to discover
• Control plot coloring: are there cases when people might want a custom color scheme? Do they want to control the field, or also more detailed options like color list and scale function?
• The filterfields widget should be extended to show the current default value when it first appears. We don't want toolbar buttons for every possible display option, but we can improve the widget for the cases where it is appropriate.

Knowing which settings are most important to change would help create focus on the most useful and user-friendly options.

[abought]

In the current form this is a very unbounded problem. It might make more sense for individual consuming sites to expose the options relevant to their specific usage. Closing this form of ticket as it is too broad to be directly actionable for the most common use cases.