engineexplorer: Display lists as expandable trees

QVariantList and QStringList are now displayed as tree entries, with
one row per element. Types of individual elements are now also displayed
on their rows. Total list length is shown as its "Value" column.

Test plan:

- child items do not appear for non-list data.
- context menu for expanded list rows is the same.
- when items are removed from list-valued data, corresponding rows
  disappear from the table.