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 537fb856, as unobtrusively as
possible, under the channel switcher preferences tab.

Closes #1232