Fix new button appearance in already shown generic wxInfoBar

If the info bar is already shown, newly added buttons have to be relaid
out to appear correctly, so just add a call to Layout() in this case.

This makes the generic version work as the native GTK one, which already
handled this use case correctly.

Closes https://github.com/wxWidgets/wxWidgets/pull/1441

Closes #14120.

src/generic/infobar.cpp

@@ -275,6 +275,8 @@ void wxInfoBarGeneric::AddButton(wxWindowID btnid, const wxString& label)
 #endif // __WXMAC__
 
     sizer->Add(button, wxSizerFlags().Centre().DoubleBorder());
+    if ( IsShown() )
+        sizer->Layout();
 }
 
 size_t wxInfoBarGeneric::GetButtonCount() const