ImageJ2Options: tweak formatting
ImageJ2Options: change new-style I/O field name

By changing the name, we change the persist key, which will cause
existing installations to reset to the default value, which is
now false. The effect will be that everyone who updates will have
their "Use SCIFIO when opening files" setting reset to false,
which will sidestep the problems people have been having with it.
We can consider reenabling SCIFIO later, after all the currently
known bugs and performance issues have been resolved.