i3-input: Proper position in non-standard cases. (#2313)

i3-input: Proper position in non-standard cases. (#2313)

This commit fixes two issues:
* We detect the EWMH support window from the root window. If this window
  currently has the input focus, we ignore this. We do this because this
  window is not a window the user is aware of and positioning relative to
  it makes no sense.
* We also detect whether the current input focus is in an i3-frame window.
  This can happen, e.g., when selecting a parent (split) container. Since
  frame windows are direct children of the root window, we must not
  translate its coordinates or we get weird results and i3-input ends up
  off-screen (see #2312). For all other windows, including those without
  any WM_CLASS, we proceed as before.

fixes #2312