RFR (XS): 6417721: Thread list should not allow multiple selection

[Erik Gahlin]

Please review a small change.

Description:
If several threads are selected in the Threads list in the Threads tab, 
some thread info is still shown in the thread info panel.

Solution:
Only allow one thread to be selected at a time, so thread info reflect 
list selection.

Bug:
http://monaco.us.oracle.com/detail.jsf?cr=6417721

Webrev:
http://cr.openjdk.java.net/~egahlin/6417721/webrev/

Testing:
Verified that only one thread can be selected.

A sponsor is needed.

Thanks
Erik