mDNS Discovery of Gamestream hosts

This pull requests adds mDNS detection of Gamestream hosts, as well as a UI element in the host entry screen to easily choose a detected host. 

mDNS detection of hosts is performed via [JmDNS](http://jmdns.sourceforge.net) (pure Java mDNS library, added as a dependency). Detected hosts are included in a drop down menu on the MainFrame, beneath the host text box. Selecting a host populates the box automatically. All other connection flow is unchanged.