Force new splits into their own new window when Option key is held down.

I wanted to resubmit this patch that adds a new feature to the Sessions window. If a user holds down the Option key while pressing either of the Split buttons, then the selected sessions will open in a new window split in the requested way. I use this nearly each time I use iTerm. If there are required changes, let's discuss them here and alter the pull request as needed. Thanks.