Issue #12141: Update existing filter when converted

This patch attempts to resolve issue #12141 (closed).

Certain tools like Levels and Brightness-Contrast allow you to convert it to another filter live. This happened by creating a new tool, so if you were editing an existing filter, the connection was lost and duplicate filters were created.

This patch copies any existing filter to the new tool and updates the name and icon to the new filter type when saved.