[Icon Item] Treat sources starting with a slash as local file

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.

Differential Revision: https://phabricator.kde.org/D9812