Enable selecting multiple files with file chooser widget.

pqFileChooserWidget has a forceSingleFile ivar that declares whether it
was appropriate to select multiple files.  Previously, you were only
able to select multiple files that were grouped by the file browser.
This change allows you to select multiple files either in groups or hand
selected by the user or any combination thereof.

Change-Id: I9d9a14585023e199cf73bd3642466319d455334b
Merge remote-tracking branch 'stage/13374-slac-multiple-modes' into 13374-slac-multiple-modes

* stage/13374-slac-multiple-modes:
  Enable selecting multiple files with file chooser widget.

Change-Id: Ib79a749a958f652a7cf1ea186eb4905e528801c2
BUG #13374. Added UI elements to support phase/scale for SLACReader.

Added pqListPropertyWidget which makes is possible to show widgets to edit
PhaseShift and FrequenceScale for SLACReader. Bring in changes to VTK that adds
support for the same on the SLACReader.

Change-Id: Ie81d5fee9e61e125dd7af95fdf41c3f448cbae62
Merge topic '13374-slac-multiple-modes' into next

bf7d145 BUG #13374. Added UI elements to support phase/scale for SLACReader.
0dfd837 Merge remote-tracking branch 'stage/13374-slac-multiple-modes' into 13374-slac-multiple-modes
b722b30 Enable selecting multiple files with file chooser widget.