Add collapsible tree views for hierarchical lists

[jan-auer]

Long lists like tests or coverage reports can easily get confusing. It would be great to have a tree view just like in file browsers, to collapse layers and view aggregated data on parent nodes.

• Group tests by folder and file name or class name
• Group coverage by folder
• Group artifacts by type (if there are many artifacts and shared types)

[dcramer]

This is done for coverage, and test groups are now aggregated by their name.