refactor(ui): "follow measure" placeholder should be migrated to default option

[johallar]

Instead of having "follow measure" as a placeholder and implicit default value, it should be an actual select option (selected by default)

Reference: #477 (comment)

[cmatzenbach]

Actually I was looking into this to fix it, and I think it's correct. The default value tells it to follow the flow measure set to the left, which it does do upon investigation - changing Flow Measure automatically changes the Bar Labels as well. So I think this should be fine as-is.

[johallar]

This is a UX issue IMO, the issue being if you have an option selected then the "Follow measure" default is not visible/selectable, so it's not obvious how to get back to the default state "Follow measure". The behavior is correct, but I think the UX could be improved by making it a real non-clearable option, instead of using placeholder text for this.

[cmatzenbach]

Yeah thinking about it, that's a good point. That should be a selectable option instead of a default, especially since you have no way of going back to it if you select something else.