[INTERNAL] moved Pref.Constants back to Eclipse

Most of the options are Eclipse specific and do not belong into the
core. 

Suggestion:

core = PreferenceConstants
eclipse = EclipsePreferenceConstants extends PreferenceConstants

Change-Id: I77d443e82271eb56fbe0931358039d5251af4e80
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1652
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>