(bug) Fixture Groups corrupted on fixture list import

[Ben Buchwald]

When a fixture list is imported, the ids of the fixtures in it get reassigned, but the groups are then created containing the fixtures with the same ids as when the file was saved, not the new ids of the fixtures it used to contain. As an example, to an empty project add 3 fixtures. Create a group with those 3 fixtures. Export it as a fixture list. In a new empty project create one fixture, then import the fixture list. The group that is created instead of containing the 3 fixtures from the fixture list, will contain the first 3 fixtures in this new project, i.e. the new fixture and the first 2 fixtures imported from the list.