v2.9.0 add optional panel implementation based on posframe.

- see the implementation in zero-input-panel-posframe.el
  this is not load by default by zero-input.el.

  the old panel is built using C and gtk, which doesn't work well on Wayland
  display server. posframe panel works on both Xorg and Wayland.

  the posframe UI looks poor cause I haven't worked on make it pretty yet.
  the code is published so I have a panel that works in Wayland.

- Makefile: do not warn about docstring longer than 80 chars when byte-compile
- Makefile: auto detect s library path
- fix a few docstring single quote escape issues.
- bugfix: define-minor-mode uses keywords args instead of obsolete positional
  args.
- bugfix: use after-focus-change-function when it's available, fallback to
  focus-in-hook and focus-out-hook when it is not.