ImageJ2Options: add description of SCIFIO param
ImageJ2Options: use primitive boolean param

There is no reason to use the wrapped Boolean type, since the value
can never be null (nor would null signify anything in this case).
ImageJ2Options: use a better name for I/O option

Firstly, the option is not limited to SCIFIO. It actually toggles the
new-style ImageJ2 I/O, which includes but is not limited to SCIFIO.

Further, writing "isUse" rankles me. Yes, it is best practice for
boolean accessors to begin with "is", but that assumes the variable is
named in a nounish way rather than a verbish way. To do it 100%
correctly means choosing a variable name to which "is" can be prepended
sensibly (e.g., "scifioEnabled").
ImageJ2Options: remove unused & dangerous constant

String constants get inlined, which means that once a public one is
declared, for backwards compatibility, it should never be changed. But
we don't really want people using this constant to access the options
field by name anyway -- we have an accessor method for that. So let's
get rid of the constant before it does too much damage downstream.
WIP: ImageJ2Options: add option for usage statistics

For privacy reasons, there must be an option to opt out of the
anonymous usage statistics data collection.

Note that this commit does not _actually_ toggle the behavior.
And actually, this option should go into a separate dialog box, since
we want to provide the option for more than just the ImageJ 1.x UI.