Adds a common widget for SVG sources, with a tool button with
some handy options:
- select file (old behaviour), pick a file from disk
- embed file (pick a file from disk, is embedded into project/symbol)
- extract embedded file (for embedded files, allows you to save these
back to a disk based svg file)
- from url (opens a dialog prompting for a url, exposing
the previously hidden functionality that svgs can be retrieved
from a remote url (eg github))
Sponsored by SMEC/SJ
