REComposeViewController: use keyWindow instead of delegate.window

I'm working on a project, where delegate.window isn't set.
UIApplication.keyWindow should be more generic & work for everybody.

Thanks for cool pod, really useful & convenient!
Version bump (2.1)
Rename placeHolderLabel to placeholderLabel
Merge commit '11325d91c0b277bdd6622d7611e4e1d2fb6e1220'
Update readme
Version bump (2.1.1)