- Rename getJavaUIPluginId() to getPreferenceNodeId() and
setJavaUIPluginId() to setPreferenceNodeId()
- Change the field underlying these methods to better reflect its usage
- The preference node id returns true if it has been set

Change-Id: Ic2490b64d09d864cc29930c943178066c80a60e3
Signed-off-by: Roland Grunberg <rgrunber@redhat.com>