JuceDemoPlugin need small update

getInputChannelName(), isInputChannelStereoPair(), etc. are marked as depreciated, but the JuceDemoPlugin is still overriding them

Thanks, yes, we'll tidy that up!

it should also have JucePlugin_IsSynth defined to 1

I think it was deliberately set to false so that the plugin would appear as an effect rather than a synth in hosts that make a distinction. But we're adding more new plugin demos, and now have both synth and effect examples. Hopefully we'll ditch the creaky old plugin demo at some point soon.