Display BUFFER in a new frame.
This works by calling `pop-up-frame-function'. If successful,
return the window used; otherwise return nil.
If ALIST has a non-nil `inhibit-switch-frame' entry, avoid
raising the new frame.
If ALIST has a non-nil `pop-up-frame-parameters' entry, the
corresponding value is an alist of frame parameters to give the
new frame.