New field controller

@webflo
It looks to me like yourself and jhedstrom are to 2 most actively working on Spaces D7 so I have decided to post my work in your sandbox. I chose your sandbox over jhedstrom because I saw you replied to another issue so I knew you were watching your sandbox.

One thing I need from spaces 6.x that has changed a lot in D7 is that it makes forms simpler for OG by hiding the form fields and setting the values based on the preset. With everything moving to fields I decided to look into creating new field controller.

This new controller adds a new set of preset values for fields instead of mixing them with variables like in D6. If a field is found then the form gets hidden and the value set according to the spaces preset value. Because this is not limited to just og fields, this would also open up possibilities for setting other field values using spaces.

I have altered the default spaces_og presets to show how the idea works.

When you have time please have a look and let me know what you think. If you decided to commit this to your sandbox, time permitting I will next try to look into remove everything from spaces_og this would replace.

Please note that I have not worked much with D7 or the new Fields and Entity system so this was a lot of trial and error for me. Based on my limited testing it all seems to work.

I have not yet learned how to use Git so I created this patch with KDE's Kompare, I hope that is ok.