Implemented ConvertToEnumAction.

This is a refactoring action designed to fix https://bugzilla.xamarin.com/show_bug.cgi?id=10351
Not sure if this works 100% in all cases. I've tested and at least basic usages work. I don't know how to write NRefactory unit tests for actions with Global Operations.
I can try adding more unit tests later if needed, but I wanted to pull this version so it can at least be reviewed/discussed.