Add support for vim's popup window

[kshpytsya]

Please consider adding support for vim's popup window which is an alternative to neovim's floating window mechanism.

https://github.com/vim/vim/blob/master/runtime/doc/popup.txt See also: https://github.com/vim/vim/issues/4063

[Shougo]

I don't want to add the feature.

Because Vim's popup window feature is not same with floating window feature. Popup window does not support window enter. ddu-ui-ff depends on the behavior. So I cannot implement it.

Popup window is built for completion window like UI.

[Shougo]

See also: https://github.com/vim/vim/issues/4063

Have you read the discussion entirely??

[Shougo]

If you really need the feature, please see this.

https://github.com/vim/vim/pull/7555

The feature must be included. It is Vim's implementation issue instead.