NetBeans implementation of Preferences API

Provide NetBeans specific implementation of Preferences API means:

implement java.util.prefs.PreferencesFactory
install this implementation in place of the platform-specific default implementation

Implementation

Implementation of NetBeans-dependent backing store involves making following decisions:

files layout definition - I suggest to keep layout used by default implementation on Linux where every preferences node stores all its key-value pairs logically grouped together into one file. This would ideally(not necessarily) mean one file for every module that uses Preferences API in NetBeans. As a storage would be used SystemFileSystem (/preferences/$absoluetPath/prefs.properties). Preferences API is considered back-end neutral and thus nobody should consider this layout to be public API that can't be changed in future.
choose format of files - properies files
the way how to registr default values - META-INF/netbens/preferences/$absolutePath/prefs.properties populated with key-value pairs as defaults.

Implementation of stored defaults (follows documentation of Preferences API):
When a program writes a value that was previously covered by an existing stored default, this default value is overridden by the new value. However, the stored default itself is not changed—it is merely shadowed by the value that was explicitly added. When a key/value pair is removed for a key that has a stored default, this default value is uncovered by this removal.

There is no known requirement for system preferences Preferences.systemroot(). So, implementation of system preferences will emulate state as if backing store was unavailable. This means that such preferences won't be persistent and methods that throw BackingStoreException (like flush(), sync()) will throw it uconditionally. Even if default implementation was used for storing system preferences there is no guarantee that these preferences will be flushed on disk because very probably user won't have sufficient rights especially on Unix.

Implementation of Preferences API won't expose any additional API nor SPI. There is no need to declare any dependency to be able to use NetBeans specific implementation of Preferences API.

Installation

There is important to avoid loading of Preferences class before installing NetBeans implementation. The best I think would be to do it even before module system is initialized. The best candidate would be Main class in org.netbeans.core.startup. I would put then implementation of preferences either in org.netbeans.core.startup or into org.netbeans.core (then CoreBridge would be used)