BUG #14956: Stop accidental selection clearing.

pqMultiBlockInspectorPanel was accidentally clearing the selection when
the active source changed. Fixed that. This was causing BUG #14956.

Change-Id: Ic3fe97b4ea5567e8f078ed443d917840ceb1dfe6
Merge topic '14956_fix_extract_selection' into next

9aa80f3f BUG #14956: Stop accidental selection clearing.