Rearranging multiple selected layers loses selection

[Keavon]

This is a regression that was broken in 7b28a17 back on August 20. The test called move_selection was broken at the time so it didn't catch it, but it was just fixed in #418 which revealed this regression from months ago. The test has been disabled, so fixing this issue should involve un-commenting that test.

To reproduce: make a bunch of layers, select a group of three contiguous ones, then use Ctrl[ or Ctrl] to rearrange them. This results in the operation succeeding, but only one layer ends up selected after that.