Named FastFeatureDetector::Type to allow its use in makePtr.

It has actually worked before (likely because it's inside a class),
but C++03 does disallow unnamed types as template arguments,
and there's a report that at least in one version of GCC it's
enforced: <http://code.opencv.org/issues/3278>.
Merge pull request #1543 from SpecLad:name-enum