Allow both System and User settings

[ctrueden]

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

[dscho]

I actually like to use the Java preferences API and not take explicit control where the data are persisted.

As to ImageJ.cfg: this is a launcher-specific file, not a user-specific file. IOW you can have different ImageJ.app/ directories with completely different ImageJ.cfg files; I use that feature myself.

[hinerm]

As we update the build system the scope of this issue will move beyond ImageJ.cfg (which should go away).

We want the ability to designate a system and user jar/plugin location. The user should probably have the ability to toggle between these on restart (so the presence of a system installation doesn't supercede the user's).