Don't lowercase filenames on Linux before opening the file

This fixes https://jira.sil.org/browse/BL-828 "[Linux] Vaccination Shell
has lost the License info for Front Cover Image".  This inspired me to
go looking for similar problems elsewhere in the Bloom code.  The actual
fix for the code is in ImageUpdater.cs.
I think you're trying not to rock the boat :-) but how about we just define a camel casing standard and stick with it? So we'd have "editOriginalMode.css", "previewMode.css", etc.