Update imagePath to prefer theme, then app, then core images

imagePath updated so that image searches follow this priority:

1) /themes/$theme/apps/$app/img
2) /themes/$theme/$app/img
3) /themes/$theme/core/img
4) $appPath/img
5) /$app/img
6) /core/img

For each folder:
- if the specified file exists, use it.
- otherwise, if $basename.svg does NOT exist, try $basename.png
(This might better be "if filename was an svg, try the png"...)