ImageJ: remove deprecated static methods

This change replaces the remaining uses of those methods with proper
context-specific calls. Yay!
Rename imagej.ImageJ to imagej.Context

This will pave the way for a new, better imagej.ImageJ.
Add an ImageJ convenience class

This serves as the top-level entry point into ImageJ2, making it easier
to work with core services while retaining extensibility (i.e., access
to third-party services).
Context: eliminate bogus generic parameter
Main: use ImageJ convenience class
InteractiveUploadTest: clean up style
Merge branch 'ij-convenience'

This branch reworks the class imagej.ImageJ to be more convenient.

The old imagej.ImageJ was renamed to imagej.Context, and has much of the
same functionality (though all the static deprecated methods have
finally been removed).

There is now a new imagej.ImageJ, part of ij-app, that includes easy
access to all core services via short method calls. The imagej.Main
class has been updated to reflect this new ease of access.

The hope is that this change saves people from having to think too hard
about "services" in some common cases.