Add the option to use a folder of gamecube saves instead of a memcard image [issue 6599]

Delroth requested this feature back in sept as [issue 6599](https://code.google.com/p/dolphin-emu/issues/detail?id=6599)
This includes the feature to create a virtual memorycard from a folder of gci files.
The saves that have been written to during gameplay will be stored in the savestate.

On first use of the option the users memorycard will be migrated to a folder (leaving the original memcard untouched)
After the memorycard folder exists, the user is required to synchronize the folder with a memcard image if wanted

commit includes the changes from PR 226
I can't really come up with anything better in this context either. I suppose it would be better to have this as a checkbox, but adding such a checkbox is also a mess with the current dialog layout. So, whatever.

Either way, I suppose calling it "GCI folder" is descriptive enough for anyone who is interested in this feature in the first place.