Service selection: Move selected services to the top

[gtk-grafana]

When adding an include filter on the service/ initial label selection scene, there's no visual indication that adding a service via the combobox, or the add to filters button does anything:

If we applied include filters with the same label as the user's active tab to the volume query, every include would filter out every other result, and users wouldn't be able to select multiple values for the same label.

So, we should visually differentiate these services (more obviously then changing the + icon to -), by either changing their background color, or putting selected label values in a new section.

[gtk-grafana]

@zizzpudding this came from @matyax in the dogfooding session today, would love to mock something up for this!