Add a way to remap user property flags in users gobject_core

[thiblahute]

For example in GStreamer we have specified some flags: https://gstreamer.freedesktop.org/documentation/gstreamer/gstparamspec.html?gi-language=c#constants

Now in the gst::element proc macro we should be able to specify those flags directly like:

#[gst::element(....)]
mod impl {
     struct MyElement {
        #[property(get, set, mutable_ready, controllable)]
        uri: Mutex<String>,
    }
}

but I haven't been able to figure out how to do that cleanly.

[thiblahute]

And if we want to go fancy mutable_XX and controllable should imply set