|Changes:||Who: Oliver Drotbohm
Why: #888 - Upgrade to Spring Plugin 2.0 snapshots.
Who: Oliver Drotbohm
Why: #831 - Polishing.
Significant rework of the nullability annotation introduction to not produce any warnings anymore. Added nullability annotations where still missing. Removed the ones that were added erroneously.
Tightened nullability contracts in a couple of places, most prominently LinkRelationProvider, that now uses a LookupContext as plugin delimiter so that implementations can selectively be looked up for item or collection link relation lookup.
Removed obsolete declaration of Findbugs dependency.