NEXUS-5472: modifier methods renamed to more appropriate ones.

And other smaller fixes.
NEXUS-5472: Javadoc
NEXUS-5472: Added UT for WritableEntrySourceModifierImplTest

While doing UT, simplified WL matcher a lot, and also modified
and simplified the modifier itself too (was trying to be too smart)
NEXUS-5472: Renamed things to better match their purpose