Disable ImageJ2 syncing by default

This branch adds a toggle for this syncing, which is now disabled by default, to avoid performance problems for regular users of ImageJ 1.x functionality, while still allowing interested people to turn on the syncing to try out new ImageJ2 features as they mature.

The syncing of ImageJ1 data structures to ImageJ2 causes a substantial performance penalty, due to limitations and bugs in the ImageJ2 data structure classes. In particular, as soon as an IJ2 `ImageDisplay` is created, it attempts to render a `ScreenImage` at full resolution even though the display is never shown onscreen. While we certainly can (and need to) fix that issue, in the meantime, let's disable ImageJ2 syncing.

This branch fixes [Fiji bug 899](http://fiji.sc/bugzilla/show_bug.cgi?id=899).