Auto-Open to the default Plugin Directory

[donovankeith]

Using the Prototype Converter it will pre-fill the Plugin Directory with a useful starting point:

However, if I click on the ... button, it shows me:

Ideally, it should open to the same directory as what's been proposed (if it exists), and if not, it should open to the plugins directory.

[NiklasRosenstein]

I can give the c4d.storage.LoadDialog() function a default path, but it does not seem like it actually uses that path. Not sure if that is a bug in the Python API or just how these file/directory opening dialogs work.

Same goes for #21