Fix for DEV-41388 (SNOW-307: Linux viewer builds renames gstreamer plugin to quicktime.)

Of the four options suggested in the jira, this is a modified version of #2.  Since the mime_types.xml file actually gets localized (it contains a bunch of user-visible text in addition to the mappings), I don't think renaming it in viewer_manifest.py will work.

Instead, I've created additional files (mime_types_linux.xml and mime_types_mac.xml) in the same location, and changed the code that loads the file in LLAppViewer::init() to look for different names per-platform.

I'm sure there are cleverer solutions (such as abusing the translation xml overlay system to create files that have deltas per platform), but those don't fit in the time-box allocated to this issue.