LibGUI: Don't update windows that aren't visible (#1410)

Because the ID of a hidden window is 0, the window server will
fail to update them when the system theme is changed. This
manifests when an application has multiple windows, some of
which are hidden, and the system theme is changed (see
https://github.com/SerenityOS/serenity/issues/1378).

This PR changes the window code to ignore update messages if
the window has the ID 0--is hidden.

Ideally the window ID would not change, and visibility would be
managed separately.

Libraries/LibGUI/Window.cpp

@@ -348,6 +348,8 @@ void Window::update()
 
 void Window::force_update()
 {
+    if (!this->is_visible())
+        return;
     auto rect = this->rect();
     WindowServerConnection::the().post_message(Messages::WindowServer::InvalidateRect(m_window_id, { { 0, 0, rect.width(), rect.height() } }, true));
 }
@@ -633,8 +635,8 @@ void Window::schedule_relayout()
 
 void Window::update_all_windows(Badge<WindowServerConnection>)
 {
-    for (auto* window : *all_windows) {
-        window->force_update();
+    for (auto& e : *reified_windows) {
+        e.value->force_update();
     }
 }