Memory Leak of FeatureEvents in Collection and performance issue

[fgdrf]

If running uDig of a long time with a lots of feature events for different layers, the collection increases constantly of time. There is a clean() call in add(index, object) that clear the cache of amount of features it larger than 10.

udig-platform/plugins/org.locationtech.udig.project/src/org/locationtech/udig/project/internal/impl/LayerImpl.java

Line 1566 in c0990a6

return super.add(arg0);

Due to implementation of EDataTypeUniqueEList and javadoc givs a hint that it also has significant performance impact