Add protocol handler support

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.

misc/setup/ioquake3.desktop

@@ -1,9 +1,10 @@
 [Desktop Entry]
 Name=ioquake3
-Exec=ioquake3
+Exec=ioquake3 --uri %u
 Icon=quake3
 Type=Application
 Terminal=false
 Encoding=UTF-8
 Categories=Game;ActionGame;
+MimeType=x-scheme-handler/quake3;
 X-SuSE-translate=false