Fixed text corruption and loading crash (take 2)

- Moved all of the operations requiring access of MLIFont objects to the
UI thread, and added checks in MLIFont methods to ensure that they're
always called only on the UI thread.  Not sure why, but even with a
thread lock pattern, I still saw the crashes.

- Also switched the check for the UI thread in Image to use this new
method.

- Finally, added the CaseCreator directory to .gitignore so I can stop
having to uncheck all of those files every time I want to check
something in while I work on the case creator.