The set_parent used to happen in the main window's `show` handler but at
that point the main window isn't mapped yet under Wayland (as there's no
buffer attached):

  [3694190.560]  -> xdg_toplevel@46.set_parent(nil)
  [3694198.318]  -> xdg_toplevel@53.set_parent(xdg_toplevel@46)
  [3694251.530]  -> wl_surface@48.attach(wl_buffer@60, 0, 0)

Conformant wayland compositors should ignore the `set_parent` in that
case. Citing the xdg-shell's set_parent() documentation:

  > Only mapped surfaces can have child surfaces. Setting a parent which
  > is not mapped is equivalent to setting a null parent. If a surface
  > …

Thus the default browser dialog would appear below the main window (but
still grabbing all input hence making epiphany look "frozen").

Moving the gtk_window_present() into idle fixes this.

Closes: World/Phosh/phoc#367