Improve display of checkbox aggregate data in search forms

Clients have asked for improvements to the way aggregation data for a multilevel vocabulary appears in a search form.

The request we have seen most is the ability to see aggregation info separated by level. Currently we see values ranked in order of hits with no attention given to the primary selection of the value:

[x] Asia/China (count)
[x] South-america/Chile (count)
[x] Europe/Italy (count)
[x] Asia/Japan (count)
[x] Europe/France (count)

Clients who consider results hierarchically would prefer something like

├── Asia (total count)
│   ├── [x] China (count)
│   └── [x] Japan (count)
├── Europe (total count)
│   ├── [x] France (count)
│   └── [x] Italy (count)
└── South-america (total count)
    └── [x] Chile (count)