qtFred feature request: Rotate around multiple ships.

We can likely piggy back on the code for rotating around a single ship, but we just have to calculate and store a different center point.

Start by locating qtFred's existing single-ship rotation implementation and trace how its center point is calculated and stored. Determine how multiple ships should contribute to that center, then verify that qtFred rotates around the resulting shared point for the requested ship group.

Written by the indexing model from the issue text.