Support passing per-plugin properties to PeasExtensionSet
Currently when using PeasExtensionSet, the set of construct-only properties passed to an extension when one is constructed cannot be modified - that is, the same construct-only properties are set for each extension instance constructed.
I have a use case for passing in a construct-only property to an extension that differs for each plugin. I can implement this via a work-around that declares a writable property on the extension interface and setting it in a
extension-added callback, but that means the extension could also modify it, which is undesirable.
One way to implement this would to support passing a Peas-specific callback as a property value, then calling that when constructing the extension and passing the result of the call as the property value.