GWindow: Make destroy-on-close an optional behavior.

Also add missing implementation of GWindow::is_visible().

LibGUI/GWindow.cpp

@@ -47,6 +47,7 @@ void GWindow::close()
 {
     if (should_exit_event_loop_on_close())
         GEventLoop::current().quit(0);
+    if (should_destroy_on_close())
         delete_later();
 }
 
@@ -421,7 +422,7 @@ void GWindow::collect_keyboard_activation_targets()
 
 bool GWindow::is_visible() const
 {
-    return false;
+    return m_window_id != 0;
 }
 
 void GWindow::update(const Rect& a_rect)

LibGUI/GWindow.h

@@ -53,6 +53,9 @@ public:
     Color background_color() const { return m_background_color; }
     void set_background_color(Color color) { m_background_color = color; }
 
+    bool should_destroy_on_close() { return m_destroy_on_close; }
+    void set_should_destroy_on_close(bool b) { m_destroy_on_close = b; }
+
     int x() const { return rect().x(); }
     int y() const { return rect().y(); }
     int width() const { return rect().width(); }
@@ -158,6 +161,7 @@ private:
     GWindowType m_window_type { GWindowType::Normal };
     bool m_is_active { false };
     bool m_should_exit_app_on_close { false };
+    bool m_destroy_on_close { true };
     bool m_has_alpha_channel { false };
     bool m_double_buffering_enabled { true };
     bool m_modal { false };