We have a special case for sources starting with "file://" but a "/" also represents an absolute path and shouldn't conflict with icon theme names.
Kicker sets a custom image as local path and then we would end up trying to load it as a QIcon::fromTheme eventually.
This will cause the implicit size of the icon item to stay at its default as we only check a custom implicit size for a source QImage or an SVG. Moreover, this potentially introduces scaling artefacts.

Unit test still passes. Comes with a new unit test to verify that non-square images get their proper implicit size set both when loaded as URL and local path. Fails before, passes with this fix.

Kicker showing proper non-square icon again