Make sourceVariable name optional on all shadow variable annotations and add sourceEntityClass attribute

A) @Next(sourceEntityClass=Vehicle.class, sourceVariableName="visitList")

B) @Next(sourceEntityClass=Vehicle.class)
// there is only 1 ListVariable in the Vehicle class

C) @Next(sourceVariableName="visitList")
// there is only 1 ListVariable called "visitList" in the whole meta-model

D) @Next()
// there is only 1 ListVariable in the whole meta-model

// Winner
X) What do we allow? A, B, C, D
What do we promote (= docs, quickstarts)? B

Add TRACE logging message to any magic decision that happens automatically (when an optional configuration is missing).