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/evtloopconsole.h

@@ -30,6 +30,10 @@ public:
     WXDWORD MSWWaitForThread(WXHANDLE hThread);
 #endif // wxUSE_THREADS
 
+    // Return true if wake up was requested and not handled yet, i.e. if
+    // m_heventWake is signaled.
+    bool MSWIsWakeUpRequested();
+
 protected:
     // get the next message from queue and return true or return false if we
     // got WM_QUIT or an error occurred