Since Vim8 we have win_getid() and getwininfo() functions to get
information about the current window. So we can use those functions to
find out, whether the current window is a quickfix or location list
window.
This avoids using a redir() over the :ls command and trying to
manually match the string quickfix and should be faster and also be more
robust, as the redir may fail if done recursively.
fixes#1319
cebfc9a64b