Rename AppService hierarchy to AppEventService

This is in preparation for a new AppService to be introduced into the
SciJava Common framework itself, for handling things like title and
version. The ImageJ AppEventService is intended for handling the
UI-level operations of "show an about dialog box", "show the preferences
dialog box" and "quit the program with user interaction".
PivotApplication: remove unneeded instance fields
Update to use new SciJava Common AppService

The getTitle, getVersion, getPOM and getManifest methods of
org.scijava.Context were split out into a separate service, the
AppService, to make overriding their behavior easier.

ImageJ now provides its own AppService implementation (extending the
default one) which overrides the title to be "ImageJ" and the version to
use the ij-core POM rather than the scijava-common POM.
Merge branch 'clean-versions'

This updates ImageJ to use the new SciJava Common AppService, allowing
ImageJ to override the application title and version to correspond to
its values, rather than those of SciJava Common itself.

As a consequence, the version in the status bar is correct again.