simple-touch: Convert to xdg-shell

wl_shell is officially deprecated so remove support for it and
instead add support for xdg-shell.  With it, we've further:

- moved out the buffer handling into its own a distinct structure in
  case we might want to do multi-buffer rendering
- perform a redraw after we have receiving the initial configure event,
  as to draw to working area where to user can use it for receving touch
  events

Additionally we are setting an appid in case one might want to use it in
tandem with kiosk-shell as to be able to place it on a
distinct/different output.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>