VIM-171 Add support for closing all windows except current

Adds <C-W>o, <C-W><C-O> keystrokes.
VIM-171 Add support for window cycling

Adds following keystrokes:
<C-W>w <C-W><C-W> next window
<C-W>W            previous window

With number both commands go to window with specified index.
VIM-171 Add checks for that window is not null