editor-plugin: check for existence of the editor

Currently, the users that use the GUI editor have no way of checking if it is installed short of actually attempting to load it (which would be slow and cumbresome).

The plugin factory always succeeds, and rightly so, because it could still be providing import()/export() that was added with NetworkManager-1.40 or so.

Let's check the presence of editor (the GUI part) upon plugin load and indicate the results via the capabilities. Needs NetworkManager-1.52 to work properly.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/2037 network-manager-applet!148 (closed)