Close plugin's windows gracefully when stopped

When a plugin is stopped, close all windows created by that plugin.
Ensure the close event on the window can not open a new window if the plugin is being stopped.

src/openrct2/scripting/Plugin.cpp

@@ -96,11 +96,25 @@ void Plugin::Start()
     _hasStarted = true;
 }
 
-void Plugin::Stop()
+void Plugin::StopBegin()
 {
+    _isStopping = true;
+}
+
+void Plugin::StopEnd()
+{
+    _isStopping = false;
     _hasStarted = false;
 }
 
+void Plugin::ThrowIfStopping() const
+{
+    if (IsStopping())
+    {
+        duk_error(_context, DUK_ERR_ERROR, "Plugin is stopping.");
+    }
+}
+
 void Plugin::LoadCodeFromFile()
 {
     _code = File::ReadAllText(_path);