Add integration test to verify shutdown behavior

This test needs to use maven-invoker-plugin to run in a separate JVM,
because ImageJ1 calls System.exit(0) upon successful shutdown. We really
want to verify that indeed this action occurs.

Note that maven-surefire-plugin and maven-failsafe-plugin do not support
System.exit getting called from tests, so using maven-invoker-plugin is
the best way to go here.
ShutdownTest: make the tests work in Eclipse

This fix is necessary to avoid problems with the EclipseHelper.
Otherwise, the context class loader from one test -- which has been
modified by ImageJ 1.x and the legacy layer -- bleeds into the next
test, causing it to fail.

Thanks to Johannes Schindelin for suggesting the solution.
Make imagej-ui-swing non-optional

The original thinking (which we unfortunately did not write down
anywhere that we could find) was probably that depending on
imagej-legacy should not require the project to inherit the
imagej-ui-swing dependency, since it is a UI-specific component.

However, the relevant legacy code using the net.imagej.ui.swing
classes (in particular, the TextEditor of the Script Editor) was not
careful to catch all ClassNotFoundExceptions etc. in cases where the
imagej-ui-swing was not also present on the classpath.

Further, the imagej-legacy project's purpose is to manage the
interaction with ImageJ1, which is inherently an AWT & Swing based
application. So there is no advantage to restricting dependence on
Swing-based libraries anyway.

Better is to simply declare it as a normal dependency. Then if someone
wants to exclude the dependency, they can do so... at their own risk.

This change eliminates the nasty exception stack traces from the output
of the ij1-quit integration test.