Add CSS selector for expanded drawers

[Diax170]

Adds a .expanded CSS class for expanded group drawers

Partially resolves discussion #3900

[Diax170]

I also came up with this snippet, which adds on-expanded and on-collapsed config options:

void Group::show_group() {
  box.set_state_flags(Gtk::StateFlags::STATE_FLAG_PRELIGHT);
  revealer.set_reveal_child(true);
  box.get_style_context()->add_class("expanded");

  if (config_["drawer"].isObject()) {
    if (config_["drawer"]["on-expand"].isString() && revealer.get_reveal_child()) {
      pid_children_.push_back(util::command::forkExec(config_["drawer"]["on-expand"].asString()));
    }
  }
}

void Group::hide_group() {
  box.unset_state_flags(Gtk::StateFlags::STATE_FLAG_PRELIGHT);
  revealer.set_reveal_child(false);
  box.get_style_context()->remove_class("expanded");

  if (config_["drawer"].isObject()) {
    if (config_["drawer"]["on-collapse"].isString() && !revealer.get_reveal_child()) {
      pid_children_.push_back(util::command::forkExec(config_["drawer"]["on-collapse"].asString()));
    }
  }
}

but ultimately gave it up because it was too buggy. If anyone wants to get this feature added in, feel free to discuss