Added wxMessageQueue::Clear().

This method can be called from the posting thread to remove any still pending
requests if they became unnecessary.

Closes #10905.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64386 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/msgqueue.h

@@ -20,6 +20,8 @@
     should terminate as there is no other way to gracefully shutdown a thread
     waiting on the message queue.
 
+    @since 2.9.0
+
     @nolibrary
     @category{threading}
 
@@ -35,6 +37,17 @@ public:
     */
     wxMessageQueue();
 
+    /**
+        Remove all messages from the queue.
+
+        This method is meant to be called from the same thread(s) that call
+        Post() to discard any still pending requests if they became
+        unnecessary.
+
+        @since 2.9.1
+     */
+    wxMessageQueueError Clear();
+
     /**
         Returns @true if the object had been initialized successfully, @false
         if an error occurred.