POM: bump to pom-imagej 2.49

This updates the version of scijava-common to 2.31.0,
which is needed for the new App API.
Add about, prefs and quit routines for ImageJ

These were previously in the imagej-plugins-commands components, but
they are core routines for ImageJ2. Now that the ImageJApp needs to
implement about(), prefs() and quit(), it makes sense to put these
command implementations here in imagej-common along with ImageJApp.
Merge branch 'app-cascade'

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

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