ProxyPad: impl ExtManual functions as trait functions, not associated functions

This allows the same calling style on a gst::GhostPad as for event_default:

Already possible

fn sink_event(&self, pad: &gst::GhostPad, bin: &BinType, event: gst::Event) -> bool {
    pad.event_default(Some(bin), event)
}

Before this commit

fn sink_chain(&self, pad: &gst::GhostPad, bin: &BinType, buffer: gst::Buffer) -> Result<gst::FlowSuccess, gst::FlowError> {
    gst::ProxyPad::chain_default(pad, Some(bin), buffer)
}

With this commit

fn sink_chain(&self, pad: &gst::GhostPad, bin: &BinType, buffer: gst::Buffer) -> Result<gst::FlowSuccess, gst::FlowError> {
    pad.chain_default(Some(bin), buffer)
}

I think this was the original intent since ProxyPadExtManual was already declared in Gir.toml.