Allow letters to be used in the "Specify IPv4 Address:" field

Title. This allows people to connect to servers using a hostname instead of an IP from the Multiplayer menu and would save people from having to enter the console and use the connect command for them (see example gif: https://cdn.discordapp.com/attachments/270211185666686978/538561950766858240/kart0032.gif).

Doesn't affect netplay, tested by connecting to kaban.is-very-cute.moe and tyronesama.moe, both worked without issue.