Don't rely on getting WM_NULL messages in wxIdleWakeUpModule

Since the switch to using an event object for idle handling wakeup,
WM_NULLs are not being sent any longer, so wxIdleWakeUpModule didn't do
anything, resulting in pending event dispatching being paused while our
event loop was not running, as it happened, for example, while the
window was resized.

See #17579.

include/wx/msw/app.h

@@ -103,6 +103,10 @@ public:
     // use it.
     static wxLayoutDirection MSWGetDefaultLayout(wxWindow* parent = NULL);
 
+    // Call ProcessPendingEvents() but only if we need to do it, i.e. there was
+    // a recent call to WakeUpIdle().
+    void MSWProcessPendingEventsIfNeeded();
+
 protected:
     int    m_printMode; // wxPRINT_WINDOWS, wxPRINT_POSTSCRIPT