Defaults provided by modules in hook_panelizer_defaults() are overriden by panelizer

If you add a default panelizer to your own module (for instance through features), it will be overriden by panelizer, because it itself implements hook_panelizer_defaults and adds defaults for all enabled panelized entities. If the panelizer hook is run after your module's hook, it will override all the panelizer defaults of the previous modules.

The attached patch changes the panelizer_panelizer_defaults() into an alter, and checks if a default panelizer panel already exists. It only adds the default panel if it wasn't already provided.