BUG $14848: Fix multi-selection in "Blocks" widget.

You can now select multiple rows and the right click to check/uncheck
the selection.

Change-Id: Id302f586bd2c2815fd6f961af2e8e275ef1cc220
Merge topic '14848_fix_mutlselection' into next

cb849661 BUG $14848: Fix multi-selection in "Blocks" widget.