Move javadocs on event handler methods to class-level
factor out inventory logic to a method on the event; not really the
role of the event, but this is a convenient way for us to reduce code
duplication and shrink patches
Shrink ItemGlassBottle patch by using FillFluidContainerEvent.fillContainer()
Shrink ItemBucket patch using the ItemGlassBottle approach
readd fields to FillBucketEvent to avoid breaking ABI; leads to
undesirable field hiding but such is the nature of public fields
hiding the non-final 'result' field is problematic; abstract via accessors