wayland: Pass shadow width to the compositor

Use xdg_surface_set_window_geometry() to tell the compositor about the
shadow widths, this makes some gnome-shell/mutter features (edge resistance,
frames around windows in the overview, side maximization, ...) work alright
with GTK+.

In order to add this, some other places in gdkwindow-wayland had to gain
some knowledge about margins:

- xdg_surface_configure() now syncs the shadow after applying the state,
  and gdk_wayland_window_set_shadow_width() possibly reconfigures the
  window in order to preserve window geometry. This is necessary to keep
  shadows in sync with state/geometry changes, as this does not happen
  all at once.
- xdg_popups relative to an xdg_surface are shown relative to buffer
  coordinates, so the left/top margins must be added there.

https://bugzilla.gnome.org/show_bug.cgi?id=736742