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'm clear that we can afford to keep the Windows functionality of being case tolerant, since that means errors will slip past us and only show up in Linux. But if we did want this, how about doing the tolower on these guys at the checking time, rather than having to specify each one in its lower case form.