Add DICOM "header" (metadata) browser to UI

[dclunie]

To be able to inspect values of DICOM attributes that are not already displayed as text in the viewer UI, or to figure out where the displayed values come from.

Begs the question of for which of the several DICOM images used to produce the UI should be inspected - probably best to default to one but give the user the choice of a different one.

Could model the behavior after the OHIF viewer, which if I recall has this capability. Most DICOM viewers do.