Fix sending wxEVT_TEXT_ENTER when using auto-completion in wxMSW

We need to explicitly generate this event from the char hook handler as
we don't get the normal WM_CHAR for it, it is apparently intercepted by
the window proc installed by the auto-completing code, so check if
wxTE_PROCESS_ENTER is used for the text entry and call a special new
MSWProcessSpecialKey() method to do the right thing if it is.

Similarly, handle Tab presses correctly if wxTE_PROCESS_TAB is used.

Closes #12613.

include/wx/msw/textentry.h

@@ -86,6 +86,16 @@ private:
     virtual WXHWND GetEditHWND() const = 0;
 
 #if wxUSE_OLE
+    // This method is called to process special keys such as Return and Tab
+    // before they're consumed by the auto-completer. Notice that it is only
+    // called if we do need to process the key, i.e. if the corresponding
+    // wxTE_PROCESS_XXX style is set in the associated object.
+    //
+    // It is not pure virtual because it won't get called if the derived class
+    // doesn't use auto-completer, but it does need to be overridden if it can
+    // be called and the default implementation asserts if this is not the case.
+    virtual void MSWProcessSpecialKey(wxKeyEvent& event);
+
     // Get the auto-complete object creating it if necessary. Returns NULL if
     // creating it failed.
     wxTextAutoCompleteData *GetOrCreateCompleter();