Greatly increase reliability of deferred closing

It's pretty hard to find a solution that checks all of the boxes. I want
something that:

- Closes instantly when you close the editor, and in REAPER you should
  be able to instantly switch between docked and floating modes
- Where there should not be a delay in user interaction when quickly
  reopening the editor (or doing that switching thing in REAPER since that's
  the same thing)
- Where the window manager should not try to reparent the window during
  the losing process as that can cause some jarring flickering
- And, of course, there should be no weird Wine X11Drv crashes

And it should do all of that in Bitwig, REAPER, Carla, Ardour and
Renoise. Apparently it's quite the task to find an approach that checks
all the boxes there.

src/wine-host/editor.h

@@ -59,7 +59,9 @@ struct Size {
  * A RAII wrapper around windows created using `CreateWindow()` that will post a
  * `WM_CLOSE` message to the window's message loop so it can clean itself up
  * later. Directly calling `DestroyWindow()` might hang for a second or two, so
- * deferring this increases responsiveness.
+ * deferring this increases responsiveness. We actually defer this even further
+ * by calling this function a little while after the editor has closed to
+ * prevent any potential delays.
  *
  * This is essentially an alternative around `std::unique_ptr` with a non-static
  * custom deleter.
@@ -74,9 +76,11 @@ class DeferredWindow {
      * Manage a window so that it will be asynchronously closed when this object
      * gets dropped.
      *
+     * @param main_context This application's main IO context running on the GUI
+     *   thread.
      * @param window A `HWND` obtained through a call to `CreateWindowEx`
      */
-    DeferredWindow(HWND window);
+    DeferredWindow(MainContext& main_context, HWND window);
 
     /**
      * Post a `WM_CLOSE` message to the `handle`'s message queue as described
@@ -85,6 +89,9 @@ class DeferredWindow {
     ~DeferredWindow();
 
     const HWND handle;
+
+   private:
+    MainContext& main_context;
 };
 
 /**
@@ -113,6 +120,9 @@ class Editor {
      * Open a window, embed it into the DAW's parent window and create a handle
      * to the new Win32 window that can be used by the hosted VST plugin.
      *
+     * @param main_context The application's main IO context running on the GUI
+     *   thread. We use this to defer closing the window in
+     *   `DestroyWindow::~DestroyWindow()`.
      * @param config This instance's configuration, used to enable alternative
      *   editor behaviours.
      * @param parent_window_handle The X11 window handle passed by the VST host
@@ -124,12 +134,11 @@ class Editor {
      * @see win32_window
      */
     Editor(
+        MainContext& main_context,
         const Configuration& config,
         const size_t parent_window_handle,
         std::optional<fu2::unique_function<void()>> timer_proc = std::nullopt);
 
-    ~Editor();
-
     /**
      * Get the Win32 window handle so it can be passed to an `effEditOpen()`
      * call. This will return the child window's handle if double editor