POM: update to pom-imagej 2.49

This updates the version of scijava-common to 2.31.0,
which is needed for the new App API.

It also bumps imagej-common to 0.8.3.
Remove obsolete service and associated commands

The AppEventService has been deprecated, in favor of individual
implementations in App plugins. This particular functionality has
now migrated into imagej-common, as part of the ImageJApp.
Merge branch 'app-cascade'

Migrates the core about(), prefs() and quit() routines for ImageJ2
out of imagej-plugins-commands, and into the imagej-common component.

See also https://github.com/imagej/imagej-legacy/pull/88