[8u] RFR 8074286: Add getSelectedIndices() to ListSelectionModel

[Zhengyu Gu]

I would like to backport this patch to 8u. It is *not* a parity 
backport. However, JDK-8072767, which is a parity backport, has 
dependence on it. Without this patch, the new test fails.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8074286
Original patch: http://hg.openjdk.java.net/jdk/jdk/rev/5daa8ef17089

The original patch does not apply cleanly. There is a conflict in 
JList.java, due to line shifts. Resolved manually.

8u webrev: http://cr.openjdk.java.net/~zgu/JDK-8074286-8u/webrev.00/

Thanks,

-Zhengyu