Revamp of the Picker Component

[ciacob]

Description
The Picker Component needs a complete overhaul, to allow the user to:

1. further refine/configure the selected items;
2. bring up a compared configuration view, where selected items that share a certain property can be edited simultaneously.*

*This is especially useful for generators, which commonly have requests like: select the intervals you want to use, and their respective importance: with the new feature, the "importance" of each interval will show in the same list, making configuration more intuitive.

See attached specification.