Support guioption 'k' to avoid resizing window when adding tabs/vsplits/etc

[ychin]

Implement the new guioption 'k' introduced in vim/vim@8ac4415. It attempts to avoid resizing the window while adding/removing the tab bar, toolbar, vertical splits. This is similar to how MacVim / Vim behaves when in fullscreen mode, and avoids the window keep resizing itself which is counter to how other normal text editors work.

I implemented the original pull request vim/vim#703, and can submit a pull request for this. It will probably mostly be modifying MMWindowController.m's logic to support this new option.