Create configuration option to control middle click tab close.

Some might argue that this shouldn't be able to be configured as it is "common" behaviour in other programs such as web browsers, but for me it is an annoyance because for 15 years this has not been the behaviour of X-Chat.
Quite simply, this creates a configuration option for 537fb85696031182cb6a79879a0f5e7ce6374779, as unobtrusively as possible, under the channel switcher preferences tab.