lookups: selectively process `case` in the metrics window

Don't apply the `case` lookup in Metrics Window.

The idea of `case` is "select desired characters and apply the feature",
but the metrics window is not a document editor, so we try to just disable
the feature around lowercase letter(s) to ease the designer's job.

r? @frank-trampe 
And beyond we need to decide if we want this at all since it adds "magic behavior" (@davelab6).