The macOS client and server were completely unusable when run from a
terminal. They blocked forever in `[NSApp run];` which was called by
Sys_InitProtocolHandler(). `applicationDidFinishLaunching` was never
called to exit the NSApp run loop.
Use SDL's SDL_DROPFILE event to receive URLs to handle on macOS instead.
This also handles URLs while the game is running (connect to new server)
instead of nothing happening when clicking a link while the game is
running.
This lets the user click a link in a web browser to very easily join a Quake 3 multiplayer game. As browser-based matchmaking websites become more popular, this makes it a lot more convenient and simple to play Quake 3 with others.
The links have the following URI format: quake3://connect/example.com:27950. The format has been designed to be flexible to allow more types of links in the future and avoiding having to make a breaking change. At the moment, "connect" is the only supported command.
