Move ImageJService interface to net.imagej package

This is more consistent with SCIFIO, which has SCIFIOPlugin and
SCIFIOService both in the toplevel io.scif package.

In the case of ImageJ, ImageJPlugin was already in net.imagej, but
ImageJService was in net.imagej.service (and was the only class there).
Add interface for managing ImageJ UIs

This is more specific than the SciJava UIService. But at the moment, it
provides no additional API methods. Rather, it simply handles mouse
presses in the main application frame, so that the status bar can be
updated with the application details and memory use as expected.