BUG: Fix default color on first color picker dialog opening

The problem was due to the color list view that was selecting the first
color in the list on focus.
(see QAbstractItemView::focusInEvent).
By setting the current index to invalid, it prevents focusInEvent to
select the first index.
Closes #2956

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21710 3bd1e089-480b-0410-8dfb-8563597acbee