Fixed logic in `onScanAddedToEditor` so that XRF XANES spectra, if
present, have prioity over transmission spectra, as originally intended.
Fixed whitespace
added `onScanAddedToEditor()` to preselect most appropriate data source
for initial exclusive view when scans are loaded.
added method `onScanEditorCreated' and connections so that
`onScanAddedToEditor` actually gets called when a scan is loaded.