UI improvements for filtering

[cmdcolin]

• Allow users to get an idea for what types of jexl filters could be applied, by inspecting "features in view"
• Allow users to see what filters are applied to the track using a hover on the track "displayblurb" or track label. It might not be super helpful to display the raw jexl expressions in these contexts so it might be worth considering whether the jexl expressions can be "named". this could involve changing the stringArray of jexl expressions to e.g. a frozen of array of [{name:'scores>400',expr:'jexl:get(feature,"score")>400'}] cc @carolinebridge