did not allow the empty vector, it did not allow empty strings as elements,
and the strings could not contain the delimiter.
Delimiter (default: ",") gets escaped as "\,".
Backslash "\" gets escaped as "\\".
The empty string "" now represents the empty vector (previously "[]").
The string "[]" now represents a vector with one element, the empty string.
"[,]" is a vector with two empty strings.
"[\\,\,]" is a vector with two strings, "\" and ",".
M src/edu/rice/cs/drjava/config/VectorOption.java
M src/edu/rice/cs/drjava/config/VectorOptionTest.java
