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