window: Delay showing default browser dialog

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.

Moving to adwaita's bottom sheets would likely fix this as well but I think it makes sense to have the current case fixed first.

Closes: World/Phosh/phoc#367 (closed)

EDIT: I assume mutter doesn't hit this due to the gtk_shell's set_modal request but other compositors like e.g. wayfire are affected as well