Ensure that picker controls are laid out

A picker control could have a completely broken appearance if its size was
never changed from the initial size under MSW. For the picker controls inside
a sizer this was indirectly fixed by the previous commit ("Make
wxSIZE_FORCE_EVENT work if only position changed in wxMSW"), but the controls
not managed by a sizer still didn't come up correctly, so add an explicit
Layout() call to ensure they do.

src/common/pickerbase.cpp

@@ -133,6 +133,8 @@ void wxPickerBase::PostCreation()
     SetSizer(m_sizer);
 
     SetInitialSize( GetMinSize() );
+
+    Layout();
 }
 
 #if wxUSE_TOOLTIPS