Fixed access to current slideshow image using correct field in action_open

Summary:
With slideshow desktop wallpaper plugin, selecting "Open wallpaper Image" from RMB does nothing. It looks like the `action_open` method called by the RMB menu action is using an old field that no longer exists.

The patch updates the code to use the correct field.

BUG: 394035

Reviewers: #plasma_workspaces, ngraham, broulik

Reviewed By: #plasma_workspaces, ngraham, broulik

Subscribers: broulik, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12777