Refs #7148. Adding pick mode for Splatterplot

Can currently add and remove the VTK probe filter, but nothing else.
Refs #7148. Switching on orthographic projection in pick mode.
Refs #7148. Trying to hook up pick mode with coord retrieval.

This doesn't seem to work yet as I can't find the way to get the coordinates.
Also, the event filter seems to trip off if you switch away from the window
and go back.
Refs #7148. Destroy probe filter if present on view switch.
Refs #7148. Getting coordinates and passing them onto service.
Refs #7148. Fixing event filter issue.
Refs #7148. Handling possible source inputs.
Refs #7148. Fixing keyboard focus issue.
Merge remote-tracking branch 'origin/feature/7148_splatterplot_pick_mode'