Issue #8565: Show disabled Symmetry Dockable menu when no image is set

This patch attempts to implement the enhancement noted in #8565 (closed). The Symmetry dockable will now show its menu even if an image is not set. The menu will be disabled and its label will default to "None".

Currently, the code that creates the menu is enclosed in a conditional check - if the image exists. The patch moves most of the image creation code outside that conditional check, leaving only the parts that connect to the image such as signals. It also calls the set_image () function after initialization so that the disabled menu appears on start.

As a note, this code is almost identical to the 2.10 version, so if approved it could be easily ported over.