Avoid using dynamic types for ESourceEwsFolder

If we register the same type from two *different* modules, the type
system gets upset. That's precisely what happens if we enable addressbook
DRA, because the addressbook and camel modules both get loaded into the
same process.

So make it a normal static class. We do have to use g_type_ensure()
though, because e_source_get_extension() will look it up by name,
iterating over the child classes of ESourceExtension.