Add option to hide prefix while chatting

This pull adds an option to the GuiChatTC screen (the GUI when editing a tab). The option is labeled "Hide prefix while typing". With this option enabled, the prefix specified for the channel will not be shown in the chat box while typing into the channel, but it will be prepended to what text was entered when the message is sent to the server. Also, if what was typed in starts with a slash, the prefix is NOT prepended (so the user's command is executed normally).