[INTERNAL] Add Preference Pages to WidgetGallery

The PreferencePages may now be tested and amended without the need for
complete Eclipse restarts.

Change-Id: Ib7d9a6d8cae88004be581ce3015f4bfacd5d4103
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1308
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>