ref(widget-builder): Extract selection components

[narsaynorath]

This prepares us to add state so we can define progressive selection. Most of it just pulls the content from the main visualize component into a smaller subcomponent. I noticed a few things as I was doing this:

• I start passing down the spanColumnOptions when it's a span dataset because it's already sorted
• I removed the code that forces the span column options to have a number trailing item because if the aggregate is field (no aggregate) then we can actually have strings in this list
• I changed the way the default gets calculated when going from an aggregate to field (no aggregate) because the way I had it before, it was filtering the options to string tags (the dataset helper for getting table options filtered out number tags), but now we can calculate the valid columns with fieldOptions to include both strings and numbers

[nikkikapadia]

code looks good but it seems that one of your changes may have broken table widgets in the spans dataset. When you change the aggregate to count from just a field (like GRADLE_VERSION) it shows an error. I think it has something to do with handling aggregate defaults?