Display OME-XML and other metadata somehow

We want to provide a mechanism for displaying OME-XML and other metadata in a dedicated window. However, we should first consider the best way of doing so without limiting the generality of our approach. SCIFIO supports multiple data models (extensibly). At the lower level, SCIFIO simply attaches the relevant metadata object to the resultant `SCIFIOCellImg` which gets returned by the `ImgOpener`. We just need a way to expose this information via the UI somehow. Perhaps the best solution would be a `PostprocessorPlugin` that displays the OME-XML. Where such a postprocessor plugin would live, though, is not immediately obvious as of this writing.

Start by tracing how ImgOpener returns SCIFIOCellImg and how the attached metadata can be accessed. Compare that path with the proposed PostprocessorPlugin location and existing UI extension points; done means agreeing on a general metadata-display design that supports SCIFIO's extensible data models.

Written by the indexing model from the issue text.