Adds an explicit choice between SVG or raster image sources,
which allows us to clean up the configuration panel for layout pictures
by hiding options which don't apply to a certain picture source. Also permits
us to:
- Reuse the standard svg selector tree widget, which loads images
in a background thread and fixes#17061
- Uses the standard SVG and image selector line edit, which permit
drag and drop of images and expose options to embed images
and link to online sources
Ultimately this is motivated by a desire to allow users to embed
images in layouts and layout templates
Sponsored by SLYR
880874bfb6