Allow both System and User settings

The ImageJ2 configuration should be stored in a single folder beneath the user's home directory, according to the conventions of the operating system:
- Windows: ${user.home}\Application Data\ImageJ
- Mac OS X: ${user.home}/Library/Preferences/ImageJ
- Linux: ${user.home}/.imagej

The ImageJ.cfg file should live there, as well as all persisted properties. At the moment, such properties are scattered around by te Java preferences API; e.g., on OS X:
- ${user.home}/Library/Preferences/imagej.*
- ${user.home}/.java/.userPrefs/imagej

Lastly, there should be user plugins and jars folders located in the same place, where additional plugins can be installed.

Migrated-From: http://trac.imagej.net/ticket/1093

Start by tracing ImageJ2 configuration persistence and its use of the Java preferences API. Compare the requested Windows, Mac OS X, and Linux locations, including ImageJ.cfg, persisted properties, and user plugins and jars. Done means these items are stored together in the platform-appropriate folder without the currently scattered preference locations.

Written by the indexing model from the issue text.