Implement only, quit and cquit window commands; make splitting assistant...

Hi, now XVim is ARC-ified, I'm finally working off the official XVim branches, so I can start contributing again.

Here's a pull request to implement some missing/incomplete window commands: only, quit and cquit. 

I also made the splitting more vim-like: e.g. if you just keep doing vsplit, you keep getting editors opening on the left. For split, you keep getting editors opening below.

Cheers
--Anthony