Provide a way to force refresh of mDNS discovery

Mupeace does not refresh discovered players, requiring the use to kill and relaunch it. Please provide a refresh button or menu entry, and maybe also trigger a refresh on network change.

[eisnerd]

Services can be discovered and removed while Mupeace is running. There may be some circumstances in which this stops working or devices on which it doesn't work at all. I should test network change properly and app pause/resume. The proxy may also be a factor. A button might help get around unfixed problems with service discovery, but it ought to work fully automatically.