ImagePlus/Display mapping fixes

Updates the `LegacyImageMap` use to shadow both `ImagePlus`es and `Displays` appropriately, taking care that the appropriate cleanup is done of any shadowing objects when either base objects are deleted.

Fixes `ImagePlus`-related memory leaks, as well as making displays available for ImageJ2 commands regardless of whether images are opened with SCIFIO or IJ 1.x.